The Email module is split into three main parts:

  • SMTP server for sending and receiving [1]
  • IMAP and POP3 server to read email [2], and Sieve language to organize it [3]
  • Antispam filter, antivirus and attachments blocker [4]

Benefits are

  • complete autonomy in electronic mail management
  • avoid problems due to the Internet Service Provider
  • ability to track the route of messages in order to detect errors
  • optimized antivirus and antispam scan

See also the following related topics:

  • How electronic mail works [5]
  • MX DNS record [6]
  • Simple Mail Transfer Protocol (SMTP) [7]
  • DKIM signature [8]


Since NethServer 7.5.1804 new Email, POP3 connector and POP3 proxy installations are based on the Rspamd filter engine. Previous NethServer installations are automatically upgraded to Rspamd as described in Email module transition to Rspamd


NethServer can handle an unlimited number of mail domains, configurable from the Email > Domains page. For each domain there are two alternatives:

  • Deliver messages to local mailboxes, according to the Maildir [9] format.
  • Relay messages to another mail server.


If a domain is deleted, email will not be deleted; any message already received is preserved.

NethServer allows storing an hidden copy of all messages directed to a particular domain: they will be delivered to the final recipient and also to a custom email address. The hidden copy is enabled by the Always send a copy (Bcc) check box.


On some countries, enabling the Always send a copy (Bcc) can be against privacy laws.

If the final recipient cannot be established (i.e. the recipient address does not exist), the message is normally rejected. Sometimes (i.e. when a mail domain is migrated) it could be useful to accept it and silently deliver the message to a catch-all mailbox. This behavior can be obtained by enabling the Accept unknown recipients option.

DKIM signature

DomainKeys Identified Mail (DKIM) [8] provides a way to validate the sending MTA, which adds a cryptographic signature to the outbound message MIME headers.

To enable the DKIM signature for a mail domain, enable Email > Domains > Sign outbound messages with DomainKeys Identified Mail (DKIM).

The DKIM signature headers are added only to messages sent through TCP ports 587 (submission) and 465 (smtps).

To work effectively, the public DNS must be configured properly. Refer to the instructions of your DNS provider to run the following steps:

  1. Add a TXT record to your public DNS service provider with key “default._domainKey”
  2. Copy and paste the given key text in the DNS record data (RDATA) section

Email addresses

Each user has a personal mailbox and any user name in the form <username>@<domain> is also a valid email address to deliver messages into it.

The list of mailboxes is shown by the Email addresses > User mailboxes page. The Edit button allows disabling the Access to email services (IMAP, POP3, SMTP/AUTH) for a specific user. Messages delivered to that user’s mailbox can be forwarded to an external email address.


If the system is bound to a remote account provider and a user account is remotely deleted, the associated mailbox must be erased manually. The file system path prefix is /var/lib/nethserver/vmail/.

Mailboxes can be shared among groups of users. The Email addresses > Shared mailboxes page allows creating a new shared mailbox and defining one or more owning groups. Shared mailboxes can also be created by any IMAP client supporting IMAP ACL protocol extension (RFC 4314).

The system enables the creation of an unlimited number of additional email addresses, from the Email addresses > Mail aliases page. Each mail alias is associated with one or more destinations. A destination can be of the following types:

  • user mailbox,
  • shared mailbox,
  • external email address.

A mail alias can be bound to any mail domain or be specific to one mail domain. For example:

Sometimes a company forbids communications from outside the organization using personal email addresses. The Local network only (or visibility Internal) option blocks the possibility of an address to receive email from the outside. Still the “local network only” address can be used to exchange messages with other accounts of the system.

Differences in the new Server Manager

Please note that “Shared mailboxes” has been renamed to “Public mailboxes”. Also, if enabled from the Mailboxes page, the mail server can automatically create aliases of existing groups. A mail sent to the group alias will be copied and delivered to each member of the group.

Mailbox configuration

The Email > Mailboxes page controls what protocols are available to access a user mailbox:

  • IMAP [12] (recommended)
  • POP3 [13] (obsolete)

For security reasons, all protocols require STARTTLS encryption by default. The Allow unencrypted connections, disables this important requirement, and allows passing clear-text passwords and mail contents on the network.


Do not allow unencrypted connections on production environments!

From the same page, the disk space of each mailbox can be limited to a default quota. If the mailbox quota is enabled, the Dashboard > Mail quota page summarizes the quota usage for each user. This summary is updated when a user logs in or a message is delivered. The quota can be customized for a specific user in Email addresses > User mailboxes > Edit > Custom mailbox quota.

Messages marked as spam (see Filter) can be automatically moved into the Junk folder by enabling the option Move to “Junk” folder. Spam messages are expunged automatically after the Hold for period has elapsed. The spam retention period can be customized for a specific user in Email addresses > User mailboxes > Edit > Customize spam message retention.

The root user can impersonate another user, gaining full rights to any mailbox contents and folder permissions. The Root can log in as another user option controls this empowerment, known also as master user in Dovecot [2].

When Root can log in as another user is enabled, the following credentials are accepted by the IMAP server:

  • user name with *root suffix appended
  • root’s password

For instance, to access as john with root password secr3t, use the following credentials:

  • user name: john*root
  • password: secr3t

Users could share their mailbox (or some parts of it, folders) with selected accounts on the system. Everyone who is given access to a shared mailbox can read or delete messages according to permissions granted by the mailbox owner.

An IMAP flag named /Seen is used to mark if a message has been read or not. In a shared mailbox, each user has their copy of the messages they have read, but sometimes a team sharing a mailbox could prefer to know if a mail has already been read by someone else. To enable sharing of the /Seen flag for all shared mailboxes use the following commands:

config setprop dovecot SharedSeen enabled
signal-event nethserver-mail-server-save

Please note that changing the SharedSeen status resets the /Seen flag for all users on all mailboxes.

Public folders are created by the administrator and are usually visible to all users (or large groups). The /Seen flag is kept for each user and it cannot be shared.


From the Email > Messages page, the Queue message max size slider sets the maximum size of messages traversing the system. If this limit is exceeded, a message cannot enter the system at all and is rejected.

Once a message enters NethServer, it is persisted to a queue, waiting for final delivery or relay. When NethServer relays a message to a remote server, errors may occur. For instance,

  • the network connection fails, or
  • the other server is down or is overloaded.

Those and other errors are temporary: in such cases, NethServer attempts to reconnect the remote host at regular intervals until a limit is reached. The Queue message lifetime slider changes this limit. By default it is set to 4 days.

While messages are in the queue, the administrator can request an immediate message relay attempt, by pressing the button Attempt to send from the Email > Queue management page. Otherwise the administrator can selectively delete queued messages or empty the queue with Delete all button.

To keep an hidden copy of any message traversing the mail server, enable the Always send a copy (Bcc) check box. This feature is different from the same check box under Email > Domain as it does not differentiate between mail domains and catches also any outgoing message.


On some countries, enabling the Always send a copy (Bcc) can be against privacy laws.


The Email > Smarthost page, configures all outgoing messages to be directed through a special SMTP server, technically named smarthost. A smarthost accepts to relay messages under some restrictions. It could check:

  • the client IP address,
  • the client SMTP AUTH credentials.


Sending through a smarthost is generally not recommended. It might be used only if the server is temporarily blacklisted [14], or normal SMTP access is restricted by the ISP.

IMAP synchronization

IMAP synchronization is based on an IMAP transfer tool called Imapsync. The purpose is to migrate email messages from a remote IMAP account to a local one.

The migration is recursive and incremental and can be repeated as many times as needed. The emails will be copied locally if they do not exist on the local server.

The system administrator of the local NethServer does not need to know the password of the local user. However, the administrator has to know the password of the remote IMAP account, unless the IMAP admin authentication is implemented also for the remote email server.

If the remote IMAP server is also a NethServer, the IMAP admin user is vmail and its password can be read from /var/lib/nethserver/secrets/vmail. The username with a *vmail suffix (e.g.*vmail) and the vmail password has to be set in the IMAP synchronization panel.


List of IMAP servers with admin authentication in Imapsync documentation


All transiting email messages are subjected to a list of checks that can be selectively enabled in Email > Filter page:

  • Block of attachments
  • Antivirus
  • Antispam

Block of attachments

The system can inspect mail attachments, denying access to messages carrying forbidden file formats. The server can check the following attachment classes:

  • executables (eg. exe, msi)
  • archives (eg. zip, tar.gz, docx)
  • custom file format list

The system recognizes file types by looking at their contents, regardless of the file attachment name. Therefore it is possible that MS Word file (docx) and OpenOffice (odt) are blocked because they actually are also zip archives.


The antivirus component finds email messages containing viruses. Infected messages are discarded. The virus signature database is updated periodically.


The antispam component [4] analyzes emails by detecting and classifying spam [15] messages using heuristic criteria, predetermined rules and statistical evaluations on the content of messages.

The filter can also check if sender server is listed in one or more blacklists (DNSBL [14]). A score is associated to each rule.

Total spam score collected at the end of the analysis allows the server to decide what to do with a message, according to three thresholds that can be adjusted under Email > Filter > Anti spam.

  1. If the spam score is above Greylist threshold the message is temporarily rejected. The greylisting [16] technique assumes that a spammer is in hurry and is likely to give up, whilst a SMTP-compliant MTA will attempt to deliver the deferred message again.
  2. If the spam score is above Spam threshold the message is marked as spam by adding the special header X-Spam: Yes for specific treatments, then it is delivered like other messages. As an alternative, the Add a prefix to spam messages subject option makes the spam flag visible on the subject of the message, by prefixing the given string to the Subject header.
  3. If the spam score is above Deny message spam threshold the message is rejected.

Statistical filters, called Bayesian [17], are special rules that evolve and quickly adapt analyzing messages marked as spam or ham.

The statistical filters can then be trained with any IMAP client by simply moving a message in and out of the Junk folder. As a prerequisite, the Junk folder must be enabled from Email > Mailboxes page by checking Move to “Junk” folder” option.

  • By putting a message into the Junk folder, the filters learn it is spam and will assign an higher score to similar messages.
  • On the contrary, by getting a message out of Junk, the filters learn it is ham: next time a lower score will be assigned.

By default, all users can train the filters using this technique. If a group called spamtrainers exists, only users in this group will be allowed to train the filters.

The bayesian filter training applies to all users on the system, not only the user that marked an email as spam or ham.

It is important to understand how the Bayesian tests really work:

  • It does not outright flag messages as spam if they contain a specific subject, or sender address. It is only collecting specific characteristics of the message.
  • A message can only be flagged one time. If the same message is flagged multiple times, it will not affect anything as the dynamic tests have already been trained by that message.
  • The Bayesian tests are not active until it has received enough information. This includes a minimum of 200 spams AND 200 hams (false positives).


It is a good habit to frequently check the Junk folder in order not to lose email wrongly recognized as spam.

If the system fails to recognize spam properly even after training, the whitelists and blacklists can help. Those are lists of email addresses or domains respectively always allowed and always blocked to send or receive messages.

The section Rules by mail address allows creating three types of rules:

  • Allow From: any message from specified sender is accepted
  • Allow To: any message to the specified recipient is accepted
  • Block From: any message from specified sender is blocked

The Allow rules have higher precedence over the Block ones. As soon as an Allow rule matches, the antispam and antivirus checks are skipped, the Block rule is not evaluated and the message is accepted.


Antivirus and antispam checks are skipped if an Allow rule matches

It is possible to create an Allow or Block rule even for an entire domain, not just for a single email address: you just need to specify the domain name (e.g.

When a second level domain domain name is specified it matches also its subdomains. For instance matches itself,, and so on.

Rspamd web interface

The antispam component is implemented by Rspamd [4] which provides its administrative web interface at


For more information on Rspamd, please read the Rspamd page.

Quarantine (beta)

NethServer scans all incomaing email messages before they are delivered to the user mailbox. The messages that are identified as spam will be sent to a specific user mailbox. The purpose of this feature is to verify the email before deleting it. If enabled, a mail notification is also sent to the postmaster (root alias) for each quarantined email.


The quarantined messages can be accessed using a web mail or an IMAP account


The mailbox used for quarantine, must be able to accept spam. It should be a local shared mailbox or a user mailbox. If an external account is used, make sure the account exists on the remote server. Please make sure the quarantine mailbox has been created only for this specific purpose, otherwise the mailbox will be overloaded with unwanted spam.

Quarantine is provided by an optional module named nethserver-mail-quarantine. Once it has been installed from the Software center you must manually set its database properties.

The properties are under the rspamd key (configuration database):

  • QuarantineAccount: The user or the shared mailbox where to send all spam messages (spam check is automatically disabled on this account). You must create it manually. You could send it to an external mailbox but then make sure to disable the spam check on the remote server
  • QuarantineStatus: Enable the quarantine, spam are no more rejected: enabled/disabled. Disabled by default
  • SpamNotificationStatus: Enable the email notification when email are quarantined: enabled/disabled. Disabled by default

For example, the following commands enable the quarantine and the mail notification to root:

config setprop rspamd QuarantineAccount QuarantineStatus enabled SpamNotificationStatus enabled
signal-event nethserver-mail-quarantine-save

Client configuration

The server supports standard-compliant email clients using the following IANA ports:

  • imap/143
  • pop3/110
  • smtp/587
  • sieve/4190

Authentication requires the STARTTLS command and supports the following variants:

  • GSSAPI (only if NethServer is bound to Samba/Microsoft Active Directory)

Also the following SSL-enabled ports are available for legacy software that still does not support STARTTLS:

  • imaps/993
  • pop3s/995
  • smtps/465


The standard SMTP port 25 is reserved for mail transfers between MTA servers. Mail user agents (MUA) must use the submission port.

Special SMTP access policies

The default NethServer configuration requires that all clients use the submission port (587) with encryption and authentication enabled to send mail through the SMTP server.

To ease the configuration of legacy environments, the Email > SMTP access page allows making some exceptions on the default SMTP access policy.


Do not change the default policy on new environments!

For instance, there are some devices (printers, scanners, …) that do not support SMTP authentication, encryption or port settings. Those can be enabled to send email messages by listing their IP address in Allow relay from IP addresses text area.


The listed IP addresses are excluded from all mail filtering checks: use this feature only as a last resort

Moreover, under Advanced options there are further options:

  • The Allow relay from trusted networks option allows any client in the trusted networks to send email messages without any restriction.
  • The Enable authentication on port 25 option allows authenticated SMTP clients to send email messages also on port 25.

Sender/login match

By default an authenticated SMTP client has no particular restrictions on setting the SMTP sender address.

To avoid the unauthorized use of email addresses and the sender address spoofing, enable the Enforce sender/login match option, available with the new Server Manager UI, under Email > Relay > Configuration > Details.

If enabled, only addresses associated to the current SMTP login are allowed.

Multiple relay hosts

The new Server Manager UI allows to describe the route of an email message, by sending it through an external relay host with specific port, authentication, and TLS settings.

Create a relay host description under Email > Relay > Create relay host.

The relay host is identified by the SMTP sender address. It is possible to match the full sender address or only the domain part of it.

Custom HELO

The first step of an SMTP session is the exchange of HELO command (or EHLO). This command takes a valid server name as required parameter (RFC 1123).

NethServer and other mail servers try to reduce spam by not accepting HELO domains that are not registered on a public DNS.

When talking to another mail server, NethServer uses its full host name (FQDN) as the value for the HELO command. If the FQDN is not registered in the public DNS, the HELO can be changed from the new Server Manager.

This configuration is also valuable if the mail server is using a free dynamic DNS service.

Outlook deleted mail

Unlike almost any IMAP client, Outlook does not move deleted messages to the trash folder, but simply marks them as “deleted”.

It is possibile to automatically move messages inside the trash folder. The option is available in the new Server Manager under the Mailboxes page.

You should also change Outlook configuration to hide deleted messages from inbox folder. This configuration is available in the options menu.


Every mail server operation is saved in the following log files:

  • /var/log/maillog registers all mail transactions
  • /var/log/imap contains users login and logout operations

A transaction recorded in the maillog file usually involves different components of the mail server. Each line contains respectively

  • the timestamp,
  • the host name,
  • the component name, and the process-id of the component instance
  • a text message detailing the operation

NethServer configuration uses Rspamd as milter. It runs an Rspamd proxy worker in “self-scan” mode [19].

The key to track the whole SMTP transaction, including Rspamd decisions is the message ID header, or the Postfix Queue ID (QID). Both are available from the message source. The Message-ID header is generated by the sender, whilst the QID is assigned by the receiving MTA. For instance

Received: from ( [])
      by (Postfix) with ESMTP id A785B308622AB
      for <>; Tue, 15 May 2018 02:05:02 +0200 (CEST)
Message-ID: <5afa242e.hP5p/>
User-Agent: Heirloom mailx 12.5 7/5/10

Here A785B308622AB is the QID, whilst 5afa242e.hP5p/ is the Message ID.

Both strings can be used with the grep command to find relevant log lines in /var/log/maillog* (note the ending “*” to search also in archived log files). For instance

grep -F 'A785B308622AB' /var/log/maillog*


/var/log/maillog:May 15 02:05:02 mail postfix/smtpd[25846]: A785B308622AB:[]
/var/log/maillog:May 15 02:05:02 mail postfix/cleanup[25849]: A785B308622AB: message-id=<5afa242e.hP5p/>
/var/log/maillog:May 15 02:05:02 mail rspamd[27538]: <8ae27d>; proxy; rspamd_message_parse: loaded message; id: <5afa242e.hP5p/>; queue-id: <A785B308622AB>; size: 2348; checksum: <b1035f4fb07162ba88053d9e38df9c93>
/var/log/maillog:May 15 02:05:03 mail rspamd[27538]: <8ae27d>; proxy; rspamd_task_write_log: id: <5afa242e.hP5p/>, qid: <A785B308622AB>, ip:, from: <>, (default: F (no action): [-0.64/20.00] [BAYES_HAM(-3.00){100.00%;},AUTH_NA(1.00){},MID_CONTAINS_FROM(1.00){},MX_INVALID(0.50){},MIME_GOOD(-0.10){text/plain;},IP_SCORE(-0.04){ip: (0.22), ipnet:, asn: 14061(0.23), country: US(-0.81);},ASN(0.00){asn:14061, ipnet:, country:US;},DMARC_NA(0.00){;},FROM_EQ_ENVFROM(0.00){},FROM_NO_DN(0.00){},NEURAL_HAM(-0.00){-0.656;0;},RCPT_COUNT_ONE(0.00){1;},RCVD_COUNT_TWO(0.00){2;},RCVD_NO_TLS_LAST(0.00){},R_DKIM_NA(0.00){},R_SPF_NA(0.00){},TO_DN_NONE(0.00){},TO_DOM_EQ_FROM_DOM(0.00){},TO_MATCH_ENVRCPT_ALL(0.00){}]), len: 2348, time: 750.636ms real, 5.680ms virtual, dns req: 47, digest: <b1035f4fb07162ba88053d9e38df9c93>, rcpts: <>, mime_rcpts: <>
/var/log/maillog:May 15 02:05:03 mail postfix/qmgr[27757]: A785B308622AB: from=<>, size=2597, nrcpt=1 (queue active)
/var/log/maillog:May 15 02:05:03 mail postfix/lmtp[25854]: A785B308622AB: to=<>, orig_to=<>,[/var/run/dovecot/lmtp], delay=0.82, delays=0.8/0.01/0.01/0.01, dsn=2.0.0, status=sent (250 2.0.0 <> gK8pHS8k+lr/ZAAAJc5BcA Saved)
/var/log/maillog:May 15 02:05:03 mail postfix/qmgr[27757]: A785B308622AB: removed


[1]Postfix mail server
[2](1, 2) Dovecot Secure IMAP server
[3]Sieve mail filtering language
[4](1, 2, 3) Rspamd – Fast, free and open-source spam filtering system.
[6]The MX DNS record,
[8](1, 2) Domain Keys Identified Mail (DKIM) is an email authentication method designed to detect email spoofing – Wikipedia
[9]The Maildir format,
[10]alterMIME is a small program which is used to alter your mime-encoded mailpack –
[11]The Markdown plain text formatting syntax,
[14](1, 2) DNSBL
[16]Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will “temporarily reject” any email from a sender it does not recognize – Wikipedia
[17]Bayesian filtering
[18]The wondrous Ways of an Email