Dovecot Logging

Dovecot always logs a detailed error message if something goes wrong. If it doesn't, it's considered a bug and will be fixed. However almost always the problem is that you're looking at the wrong log file. Dovecot uses two different logs:

Info log: All informational and debug messages are logged here. It may also contain some vague error messages, which are also logged in more detail in the error log.
Error log: All errors are logged here. As long as there are no errors, nothing is ever written here.

By default Dovecot logs to syslog using mail facility using several different log levels. In many systems syslog is configured to log messages to different log files depending on the level. The levels and their commonly used log file locations are:

info: Informational and debug messages. /var/log/mail.info
warn: Warnings that don't cause an actual error, but are useful to know about. /var/log/mail.warn
err: Non-fatal errors. /var/log/mail.err
crit: Fatal errors that cause the process to die. There are no commonly used log file for these.
Often /var/log/mail.log or /var/log/maillog contains all of the above, but not always.

You can find the correct log files using these methods:

Info log: After starting Dovecot, grep "starting up" /var/log/*. It should show a line such as: Dovecot v1.0.0 starting up
Error logs: Use dovecot --log-error command. Then grep "This is Dovecot's" /var/log/* to find them. You should see:
- With Dovecot v1.0.0 you'll find only the crit log: This is Dovecot's error log
- With Dovecot v1.0.1 you'll find all the warn, err and crit logs:
  - warn: This is Dovecot's warning log
  - err: This is Dovecot's error log
  - crit: This is Dovecot's fatal log

You can also check your /etc/syslog.conf to see how it's configured. A good configuration for Dovecot could be:

mail.*                          -/var/log/mail.log
mail.warn;mail.err;mail.crit  -/var/log/mail.err

Internal Errors

If IMAP or POP3 processes encounter some error, they don't show the exact reason for clients. Instead they show:

Internal error occurred. Refer to server log for more information. [2006-01-07 22:35:11]

The point is that whenever anything unexpected happens, Dovecot doesn't leak any extra information about it to clients. They don't need it and they might try to exploit it in some ways, so the less they know the better.

The real error message is written to the error log file. The timestamp is meant to help you find it.

Changing Log File Paths

If you don't want to use syslog, or if you just can't find the Dovecot's error logs, you can make Dovecot log elsewhere as well:

log_path = /var/log/dovecot.log
# If you want everything in one file, just don't specify info_log_path
info_log_path = /var/log/dovecot-info.log

The error messages will go to file specified by log_path, while everything else goes to info_log_path. If you do this, make sure you're really looking at the log_path file for error messages, since the "Starting up" message is written to info_log_path file.

Rotating Logs

If you change from syslog to an external log file, you can use logrotate (available on most recent linux distros) to maintain the Dovecot logfile so it doesn't grow beyond a manageable size. Save the below scriptlet as /etc/logrotate.d/dovecot:

# dovecot SIGUSR1: Re-opens the log files.
/var/log/dovecot*.log {
  missingok
  notifempty
  delaycompress
  sharedscripts
  postrotate
    /bin/kill -USR1 `cat /var/run/dovecot/master.pid 2>/dev/null` 2> /dev/null || true
  endscript
}

NOTE: change the path to the logfile(s) and the master.pid file as appropriate for your system configuration.