The Email module is split into three main parts:
- SMTP server for sending and receiving 
- IMAP and POP3 server to read email , and Sieve language to organize it 
- Anti-spam filter, anti-virus and attachments blocker 
- 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 anti-virus and anti-spam scan
See also the following related topics:
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  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.
NethServer can automatically append a legal notice to sent messages. This text is called disclaimer and it can be used to meet some legal requirements. Please note signature and disclaimer are very different concepts.
The signature should be inserted inside the message text only by the mail client (MUA): Outlook, Thunderbird, etc. Usually it is a user-defined text containing information such as sender addresses and phone numbers.
John Smith President | My Mighty Company | Middle Earth 555-555-5555 | email@example.com | http://www.mydomain.com
The “disclaimer” is a fixed text and can only be attached (not added) to messages by the mail server.
This technique allows maintaining the integrity of the message in case of digital signature.
This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This message contains confidential information and is intended only for the individual named.
The disclaimer text can contain Markdown  code to format the text.
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
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:
- First domain: mydomain.net
- Second domain: example.com
- Email address info valid for both domains: firstname.lastname@example.org, email@example.com
- Email address goofy valid only for one domain: firstname.lastname@example.org
Sometimes a company forbids communications from outside the organization using personal email addresses. The Local network only 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.
The Email > Mailboxes page controls what protocols are available to access a user mailbox:
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.
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 .
When Root can log in as another user is enabled, the following credentials are accepted by the IMAP server:
- user name with
- root’s password
For instance, to access as
john with root password
use the following credentials:
- user name:
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 , or normal SMTP access is restricted by the ISP.
All transiting email messages are subjected to a list of checks that can be selectively enabled in Email > Filter page:
- Block of attachments
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 anti-virus component finds email messages containing viruses. Infected messages are discarded. The virus signature database is updated periodically.
The anti-spam component  analyzes emails by detecting and classifying spam  messages using heuristic criteria, predetermined rules and statistical evaluations on the content of messages. The rules are public and updated on a regular basis. The filter can also check if sender server is listed in one or more blacklists (DNSBL). A score is associated to each rule.
Total spam score collected at the end of the analysis allows the server to decide whether to reject the message or mark it as spam and deliver it anyway. The score thresholds are controlled by Spam threshold and Deny message spam threshold sliders in Email > Filter page.
Messages marked as spam have a special header
The Add a prefix to spam messages subject option makes the
spam flag visible on the subject of the message, by prepending the
given string to the
Statistical filters, called Bayesian , 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 or marking it as spam if your client provides such feature. As 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 to not losing 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:
- Block From: any message from specified sender is blocked
- Allow From: any message from specified sender is accepted
- Allow To: any message to the specified recipient is accepted
It’s possible to create an ‘Allow’ or ‘Block’ rule even for a complete email domain, not just for a single email address : you just need to specificy the desired domain (e.g. : nethserver.org).
Antivirus checks are enforced despite whitelist settings.
Block port 25¶
If the system is acting as the network gateway, green and blue zones will not be able to send mail to external servers through port 25 (SMTP). Blocking port 25 could prevent remotely controlled machines inside the LAN from sending SPAM.
The administrator can change this policy creating a custom firewall rule inside the Rules page.
The server supports standard-compliant email clients using the following IANA ports:
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:
The standard SMTP port 25 is reserved for mail transfers between MTA servers. On clients use only submission ports.
If NethServer acts also as DNS server on the LAN, it registers its name as MX record along with the following aliases:
- MX record:
- Available aliases:
Some email clients (e.g. Mozilla Thunderbird) are able to use DNS aliases and MX record to automatically configure email accounts by simply typing the email address.
To disable local MX and aliases, access the root’s console and type:
config setprop postfix MxRecordStatus disabled signal-event nethserver-hosts-update
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.
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.
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 public DNS, the HELO can be fixed by setting a special
prop. For instance, assuming
myhelo.example.com is the publicly
registered DNS record, type the following commands:
config setprop postfix HeloHost myhelo.example.com signal-event nethserver-mail-common-save
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’s possibile to automatically move messages inside the trash using following commands:
config setprop dovecot DeletedToTrash enabled signal-event nethserver-mail-server-save
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/maillogregisters all mail transactions
/var/log/imapcontains users login and logout operations
A transaction recorded in the
maillog file usually involves
different components of the mail server. Each line contains
- the timestamp,
- the host name,
- the component name, and the process-id of the component instance
- a text message detailing the operation
A picture of the whole system is available from workaround.org .
|||Postfix mail server http://www.postfix.org/|
|||(1, 2) Dovecot Secure IMAP server http://www.dovecot.org/|
|||Sieve mail filtering language https://en.wikipedia.org/wiki/Sieve_(mail_filtering_language)|
|||MTA/content-checker interface http://www.ijs.si/software/amavisd/|
|||The MX DNS record, https://en.wikipedia.org/wiki/MX_record|
|||The Maildir format, https://en.wikipedia.org/wiki/Maildir|
|||The Markdown plain text formatting syntax, https://en.wikipedia.org/wiki/Markdown|
|||Spamassassin home page http://wiki.apache.org/spamassassin/Spam|
|||Bayesian filtering https://en.wikipedia.org/wiki/Naive_Bayes_spam_filtering|
|||The wondrous Ways of an Email https://workaround.org/ispmail/wheezybig-picture/|