Mail servers: technical details

Dovecot

Generating passwords

Passwords are generated with the dovecot-doveadm utility:

/usr/bin/doveadm pw -s CRAM-MD5 -u <email address>

Delivering email to a user mailbox

dovecot-lda is set by the SMTP-server as the default mail transfer agent.

The utility performs the following steps:

1. When the email is sent from the SMTP-server, the utility gets the information about the mailbox from the file /etc/dovecot/dovecot-passwd.
2. The utility executes the setuid command to drill down to the mailbox owner level.
3. The utility checks the user's disk quotas:
   1. Specifies the email size in /.maildir/maildirsize.
   2. Calculates the total mailbox size.
   3. Compares it with the quota specified in dovecot-passwd. If the quota is exceeded, the email will be rejected.
4. If the sieve plug-in is connected, the utility checks that the filter file-script is present (.dovecot.sieve per default). If the file exists, it will be executed.

Checking disk quotas

Execute the command to check your disk quota usage:

/usr/bin/doveadm quota get -A

SpamAssassin

SpamAssassin settings are kept in /etc/mail/spamassassin/local.cf. The main parameters:

• required_score — what score is needed to flag an email as spam. The minimum value is 0, the maximum — 100. The default value is 5;
• rewrite_header — a header that will be added into the "Subject" field if the email is flagged as spam;
• report_contact — contact email. It is added into the spam report.