Email

The Email module is split in 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]
  • Anti-spam filter, anti-virus 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 anti-virus and anti-spam scan

See also the following related topics:

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

Domains

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 [8] format.
  • Relay messages to another mail server.

Note

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 local user (or group). The hidden copy is enabled by the Always send a copy (Bcc) check box.

Warning

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

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.

Signature example:

John Smith
President | My Mighty Company | Middle Earth
555-555-5555 | [email protected] | 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.

Disclaimer example:

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 [9] code to format the text.

Email addresses

The system enables the creation of an unlimited number of email addresses, also known as pseudonyms, from the Email addresses page. Each address is associated with a system user or group owning a mailbox (see User and group mailboxes). It can be enabled on all configured domains or only on specific domains. For example:

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.

When creating a new account from the Users or Groups page, the system suggests a default email address for each configured mail domain.

For instance, creating a new account for user Donald Duck:

User and group mailboxes

Email messages delivered to a user or group account, as configured from the Email addresses page, are written to a disk location known as mailbox.

When the Email module is installed, existing user and group accounts do not have a mailbox. It must be explicitly enabled from the Users > Services or Groups > Services tab. Instead, newly created accounts have this option enabled by default.

From the same Services page under Users or Groups it can be defined an external email address where to Forward messages. Optionally, a copy of the message can be stored on the server.

When an address is associated with a group, the server can be configured to deliver mail in two ways, from the Groups > Services tab:

  • send a copy to each member of the group
  • store the message in a shared folder. This option is recommended for large groups receiving big messages.

Warning

Deleting a user or group account erases the associated mailbox!

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

  • IMAP [10] (recommended)
  • POP3 [11] (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.

Warning

Do not allow unencrypted connections on production environments!

From the same page, the disk space of a mailbox can be limited to a quota. If the mailbox quota is enabled, the Dashboard > Mail quota page summarizes the quota usage for each user. The quota can be customized for a specific user in Users > Edit > Services > Custom mailbox quota.

Messages marked as spam (see Filter) can be automatically moved into the junkmail folder by enabling the option Move to “junkmail” 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 Users > Edit > Services > Customize spam message retention.

The admin user can impersonate another user, gaining full rights to the latter’s mailbox contents and on folder permissions. The Admin can log in as another user option controls this empowerment, known also as master user in [2].

When Admin can log in as another user is enabled, the IMAP server accepts any user name with *admin suffix appended and admin’s password.

For instance, to access as john with admin’s password secr3t, use the following credentials:

  • username: john*admin
  • password: secr3t

Messages

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.

Warning

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

The Send using a smarthost option, forces 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.

Note

Sending through a smarthost is generally not recommended. It might be used only if the server is temporarily blacklisted [12], or normal SMTP access is restricted by the ISP.

Filter

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

  • Block of attachments
  • Anti-virus
  • Anti-spam

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.

Anti-virus

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

Anti-spam

The anti-spam component [14] analyzes emails by detecting and classifying spam [13] 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 X-Spam-Flag: YES. 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 Subject header.

Statistical filters, called Bayesian [15], 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 junkmail folder. As prerequisite, the junkmail folder must be enabled from Email > Mailboxes page by checking Move to “junkmail” folder” option.

  • By putting a message into the junkmail 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 junkmail, 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.

Note

It is a good habit to frequently check the junkmail 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).

Note

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.

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:

  • LOGIN
  • PLAIN

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

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

Warning

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:

  • smtp.<domain>
  • imap.<domain>
  • pop.<domain>
  • pop3.<domain>

For example:

  • Domain: mysite.com
  • Hostname: mail.mysite.com
  • MX record: mail.mysite.com
  • Available aliases: smtp.mysite.com, imap.mysite.com, pop.mysite.com, pop3.mysite.com.

Note

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.

Warning

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.

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 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.

Email in Active Directory

The Email module integrates with an Active Directory (AD) environment, if Active Directory member role is enabled in Windows Network page.

Make sure LDAP accounts branch in Windows Network page is actually set to the LDAP branch where email users and groups are placed.

This is an example of an user entry in AD LDAP (some attributes omitted):

dn: CN=John Smith,OU=Sviluppo,OU=Nethesis,DC=adnethesis,DC=it
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user
cn: John Smith
sn: Smith
givenName: John
distinguishedName: CN=John Smith,OU=Sviluppo,OU=Nethesis,DC=adnethesis,DC
 =it
instanceType: 4
displayName: John Smith
memberOf: CN=sviluppo,OU=Nethesis,DC=adnethesis,DC=it
memberOf: CN=secgroup,OU=Nethesis,DC=adnethesis,DC=it
memberOf: CN=tecnici,OU=Nethesis,DC=adnethesis,DC=it
name: John Smith
primaryGroupID: 513
sAMAccountName: john.smith
sAMAccountType: 805306368
userAccountControl: 66048
userPrincipalName: [email protected]
objectCategory: CN=Person,CN=Schema,CN=Configuration,DC=adnethesis,DC=it
mail: [email protected]
otherMailbox: smtp:[email protected]
proxyAddresses: smtp:[email protected]

To make NethServer work with the external LDAP database provided by Active Directory, the following rules applies:

  1. Only enabled accounts are considered (userAccountControl attribute).
  2. IMAP and SMTP login name is the value of sAMAccountName attribute.
  3. Email addresses associated with an user are the values of mail, otherMailbox and proxyAddresses attributes. The last two attributes expect a smtp: prefix before the actual value. Also userPrincipalName is considered an email address, by default; this can be disabled (see commands below).
  4. A group email address is the value of its mail attribute. By default any group is treated as a distribution list: a copy of the email is delivered to its members.
  5. The domain part of email addresses specified by the above attributes must match a configured domain, otherwise it is ignored.

To configure security groups as shared folders globally, type the following commands at root’s console:

config setprop postfix AdsGroupsDeliveryType shared
signal-event nethserver-samba-save

Warning

Avoid AD group names containing uppercase letters with shared folder: IMAP ACLs does not work properly. See BUG#2744.

To avoid the userPrincipalName attribute to be considered as a valid email address, type the following commands at root’s console:

config setprop postfix AdsMapUserPrincipalStatus disabled
signal-event nethserver-samba-save

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.

Log

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

Here follows a brief description of the component names and the typical actions performed.

transfer/smtpd

This is the public-facing SMTP daemon, listening on port 25. A log line from this component identifies an activity involving another Mail Transfer Agent (MTA).

submission/smtpd

This is the SMTP daemon listening on submission port 587 and smtps port 465. A log line from this component identifies a Mail User Agent (MUA) that sends an email message.

amavis

The Amavis SMTP daemon enforces all mail filtering rules. It decides what is accepted or not. Log lines from this component detail the filter decisions.

queue/smtpd

This is an internal SMTP daemon, accessible only from the local system. It receives and queues good messages from Amavis.

relay/smtp

This is the SMTP client talking to a remote server: it picks a message from the queue and relays it to the remote server, as specified by the mail domain configuration.

delivery/lmtp

Messages directed to local accounts are picked up from the queue and transferred to the local Dovecot instance.

dovecot

The Dovecot daemon delivers messages into users mailboxes, possibly applying Sieve filters.

A picture of the whole system is available from workaround.org [16].

References

[1]Postfix mail server http://www.postfix.org/
[2](1, 2) Dovecot Secure IMAP server http://www.dovecot.org/
[3]Sieve mail filtering language http://en.wikipedia.org/wiki/Sieve_(mail_filtering_language)
[4]MTA/content-checker interface http://www.ijs.si/software/amavisd/
[5]Email, http://en.wikipedia.org/wiki/Email
[6]The MX DNS record, http://en.wikipedia.org/wiki/MX_record
[7]SMTP, http://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol
[8]The Maildir format, http://en.wikipedia.org/wiki/Maildir
[9]The Markdown plain text formatting syntax, http://en.wikipedia.org/wiki/Markdown
[10]IMAP http://en.wikipedia.org/wiki/Internet_Message_Access_Protocol
[11]POP3 http://en.wikipedia.org/wiki/Post_Office_Protocol
[12]DNSBL http://en.wikipedia.org/wiki/DNSBL
[13]SPAM http://en.wikipedia.org/wiki/Spamming
[14]Spamassassin home page http://wiki.apache.org/spamassassin/Spam
[15]Bayesian filtering http://en.wikipedia.org/wiki/Naive_Bayes_spam_filtering
[16]The wondrous Ways of an Email https://workaround.org/ispmail/lenny/bigpicture