14.7. Configuration of the mail server#

14.7.1. Configuration of a relay host for sending the emails#

By default Postfix creates a direct SMTP connection to the mail server responsible for the domain when an email is sent to a non-local address. This server is determined by querying the MX record in the DNS.

Alternatively, a mail relay server can also be used, i.e., a server which receives the mails and takes over their further sending. This type of mail relay server can be provided by a superordinate corporate headquarters or the internet provider, for example. To set a relay host, it must be entered as a fully qualified domain name (FQDN) in the Univention Configuration Registry Variable mail/relayhost.

If authentication is necessary on the relay host for sending, the Univention Configuration Registry Variable mail/relayauth must be set to yes and the /etc/postfix/smtp_auth file edited. The relay host, username and password must be saved in this file in one line: FQDN-Relayhost username:password

To adopt the changes in Postfix, complete the step with the command:

$ postmap /etc/postfix/smtp_auth

Note

To ensure an encrypted connection while using a relay host, the Postfix option smtp_tls_security_level=encrypt has to be set. Univention Corporate Server will set this option automatically, if mail/relayhost is set and mail/relayauth is set to yes and mail/postfix/tls/client/level is not set to none.

14.7.2. Configuration of the maximum mail size#

The Univention Configuration Registry Variable mail/messagesizelimit can be used to set the maximum size in bytes for incoming and outgoing emails. Postfix must be restarted after modifying the setting. The preset maximum size is 10240000 bytes. If the value is configured to 0 the limit is effectively removed. Please note that email attachments are enlarged by approximately a third due to the base64 encoding.

14.7.3. Configuration of a blind carbon copy for mail archiving solutions#

If the Univention Configuration Registry Variable mail/archivefolder is set to an email address, Postfix sends a blind carbon copy of all incoming and outgoing emails to this address. This results in an archiving of all emails. The email address must already exist. It can be either one already registered in Univention Corporate Server as the email address of a user, or an email account with an external email service. As standard the variable is not set.

Postfix must then be restarted.

14.7.4. Configuration of soft bounces#

In a number of error situations (e.g., for non-existent users) the result may be a mail bounce, i.e., the email cannot be delivered and is returned to the sender. When Univention Configuration Registry Variable mail/postfix/softbounce is set to yes emails are never returned after a bounce, but instead are held in the queue. This setting is particularly useful during configuration work on the mail server.

14.7.5. Configuration of SMTP ports#

On a Univention Corporate Server mail server Postfix is configured to listen for connections on three ports:

Port 25 - SMTP

Port 25 (SMTP) should be used by other mail servers only. By default authentication is disabled. If submission of emails from users is wanted on port 25, authentication can be enabled by setting the Univention Configuration Registry Variable mail/postfix/mastercf/options/smtp/smtpd_sasl_auth_enable to yes.

Port 465 - SMTPS

Port 465 (SMTPS) allows authentication and email submission through a SSL encrypted connection. SMTPS has been declared deprecated in favor of port 587 but is kept enabled for legacy clients.

Port 587 - Submission

Port 587 (Submission) allows authentication and email submission through a TLS encrypted connection. The use of STARTTLS is enforced.

The submission port should be preferred by email clients. The use of the ports 25 and 465 for email submission is deprecated.

14.7.6. Configuration of additional checks#

When using a mail server that is directly accessible from the internet, there is always a risk that spam sender, spam bots or broken mail servers are continually trying to deliver unwanted emails (for example spam) to the UCS system.

To reduce the load of the mail server for such cases, Postfix brings its own service with the name postscreen, which is put in front of Postfix and accepts incoming SMTP connections. On these incoming SMTP connections, some lightweight tests are first performed. If the result is positive, the respective connection is passed on to Postfix. Otherwise the SMTP connection is terminated and thus the incoming mail is rejected before being in the area of responsibility of the UCS mail server.

By default, postscreen is not active. By setting the Univention Configuration Registry Variable mail/postfix/postscreen/enabled to the value yes, postscreen can be activated.

Various UCR variables with the prefix mail/postfix/postscreen/ can be used to configure postscreen. A list of all relevant UCR variables including descriptions can be retrieved e.g. on command line via the command:

$ ucr search --verbose mail/postfix/postscreen/

Note

After each change of a UCR variable for postscreen the configuration of Postfix and postscreen should be reloaded. This can be triggered e.g. via the command systemctl reload postfix.

14.7.7. Custom Postfix configuration#

It is possible to modify the Postfix configuration, that resides within the file /etc/postfix/main.cf, beyond the variables that can be set with Univention Configuration Registry Variable.

If the file /etc/postfix/main.cf.local exists, its content will be appended to the file main.cf. To transfer changes of main.cf.local to main.cf, the following command must be executed:

$ ucr commit /etc/postfix/main.cf

For the Postfix service to accept the changes, it must be reloaded:

$ systemctl reload postfix

If a Postfix variable that has previously been set in main.cf is set again in main.cf.local, Postfix will issue a warning to the log file /var/log/mail.log.

Note

If Postfix’ behavior is not as expected, first remove configuration settings made by main.cf.local. Rename the file or comment out its content. Next run the two commands above. The configuration will then return to UCS defaults.

14.7.8. Configuring the alias expansion limit#

When sending a mail to a group including other nested groups, the mail may not be accepted/delivered. This is caused by Postfix trying to expand the number of the primary recipients via virtual alias expansion. This number is limited to 1000 users by default which might be too low.

To adjust the number to (for instance) 5000 users, the following line can be added or edited in /etc/postfix/main.cf.local:

virtual_alias_expansion_limit = 5000

Afterwards Postfix needs to be restarted:

$ systemctl restart postfix

14.7.9. Handling of mailboxes during email changes and the deletion of user accounts#

A user’s mailbox is linked to the primary email address and not to the username. The Univention Configuration Registry Variable mail/dovecot/mailbox/rename can be used to configure the reaction when the primary email address is changed:

  • If the variable is set to yes, the name of the user’s IMAP mailbox is changed. This is the standard setting since UCS 3.0.

  • If the setting is no, it will not be possible to read previous emails any more once the user’s primary email address is changed! If another user is assigned a previously used primary email address, they receive access to the old IMAP structure of this mailbox.

The Univention Configuration Registry Variable mail/dovecot/mailbox/delete can be used to configure, whether the IMAP mailbox is also deleted. The value yes activates the removal of the corresponding IMAP mailbox if one of the following actions is performed:

  • deletion of the user account

  • removal of the primary email address from the user account

  • changing the user’s mail home server to a different system

With default settings (no) the mailboxes are kept if one of the actions above is performed.

The combination of the two variables creates four possible outcomes when the email address is changed:

Table 14.4 Renaming of email addresses#

mail/dovecot/mailbox/…

Meaning

rename=yes and delete=no (default)

The existing mailbox will be renamed. Emails will be preserved and will be accessible at the new address.

rename=yes and delete=yes

The existing mailbox will be renamed. Emails will be preserved and will be accessible at the new address.

rename=no and delete=no

A new, empty mailbox will be created. The old one will be preserved on disk with the old name and will thus not be accessible to users.

rename=no and delete=yes

A new, empty mailbox will be created. The old one will be deleted from the hard disk.

14.7.10. Distribution of an installation on several mail servers#

The UCS mail system offers the possibility of distributing users across several mail servers. To this end, each user is assigned a so-called mail home server on which the user’s mail data are stored. When delivering an email, the responsible home server is automatically determined from the LDAP directory.

It must be observed that global IMAP folders (see Management of shared IMAP folders) are assigned to a mail home server.

If the mail home server changes for a user, the user’s mail data is not moved to the server automatically.

14.7.11. Mail storage on NFS#

Dovecot supports storing emails and index files on cluster file systems and on NFS. Some settings are necessary to prevent data loss in certain situations.

The following settings assume that mailboxes are not accessed simultaneously by multiple servers. This is the case if for each user exactly one mail home server has been configured.

To achieve higher performance, index files can be kept on the local servers disk, instead of storing them together with the messages on NFS. The index files can then be found at /var/lib/dovecot/index/. To activate this option, set Univention Configuration Registry Variable mail/dovecot/location/separate_index=yes.

With the above settings the mail server should work without problems on NFS. There are however a lot of different client and server systems in service. In case you encounter problems, here are some notes that might help:

The Dovecot documentation has more information on the topic: Dovecot Wiki: Mail storage on shared disks and Dovecot Wiki: NFS.

14.7.12. Connection limits#

In a default Univention Corporate Server configuration Dovecot allows 400 concurrent IMAP and POP3 connections each. That is enough to serve at least 100 concurrently logged in IMAP users, possibly a lot more.

How many IMAP connections are opened by a user depends on the clients they use:

  • Web mail opens just a few short lived connections.

  • Desktop clients keep multiple connections open over a long period of time.

  • Mobile clients keep just a few connections open over a long period of time. But they tend to never close them, unnecessarily wasting resources.

The limits exist mainly to resist denial of service attacks that open a lot of connections and create lots of processes.

To list the open connections, run:

$ doveadm who

To display the total amount of open connections, run:

$ doveadm who -1 | wc -l

The Univention Configuration Registry Variables mail/dovecot/limits/* can be set to modify the limits. The process of adapting those variables is only semi automatic, because of their complex interaction. For the meaning of each variable refer to Dovecot Wiki: Service configuration.

Dovecot uses separate processes for login and to access emails. The limits for these can be configured separately. The maximum number of concurrent connections to a service and the maximum number of processes for a service is also configured separately. Setting mail/dovecot/limits/default_client_limit = 3000 changes the limit for the maximum number of concurrent connections to the IMAP and POP3 services but does not change the maximum number of processes allowed to run. With the Univention Corporate Server default settings Dovecot runs in “High-security mode”: each connection is served by a separate process. The default is to allow only 400 processes, so only 400 connections can be made.

To allow 3000 clients to connect to their emails, another Univention Configuration Registry Variable has to be set:

$ ucr set mail/dovecot/limits/default_client_limit=3000
$ ucr set mail/dovecot/limits/default_process_limit=3000
$ doveadm reload

Reading /var/log/dovecot.info reveals a warning:

config: Warning: service auth { client_limit=2000 } is lower than required under max. load (15000)
config: Warning: service anvil { client_limit=1603 } is lower than required under max. load (12003)

The services auth (responsible for login and SSL connections) and anvil (responsible for statistics collection) are set to their default limits. Although 3000 POP3 and IMAP connections and processes are allowed, the connection limit for the login service is too low. Leaving it like this will lead to failed logins.

The values are so high, because default_client_limit and default_process_limit do not only lift limits for IMAP and POP3, but also for other services like lmtp and managesieve-login. Those services can now start more processes that have to be monitored and can theoretically make more authentication requests. This increases the number of possible concurrent connections to the auth and anvil services.

The values have to be adapted, using the numbers from the log file:

$ ucr set mail/dovecot/limits/auth/client_limit=15000
$ ucr set mail/dovecot/limits/anvil/client_limit=12003
$ doveadm reload

Another warning appears in /var/log/dovecot.info:

master: Warning: fd limit (ulimit -n) is lower than required under max. load (2000 < 15000),…
 because of service auth { client_limit }

The Linux kernel controlled setting ulimit setting (limit on the number of files/connections a process is allowed to open) is changed only when the Dovecot service is restarted:

$ systemctl restart dovecot

No more warnings are written to the log file and both IMAP and POP3 servers now accept 3000 client connections each.

Univention Corporate Server configures Dovecot to run in “High-security mode” by default. For installations with 10.000s of users, Dovecot offers the “High-performance mode”. The performance guide has further details on how to configure it, see UCS performance guide [6].