All abnormal events, downloads and deliveries are logged by MailerQ in different log files.

Error Logs

All abnormal events are logged to the error logfile. This includes failures like databases that are suddenly offline, or RabbitMQ connections that are lost. The name and error of the error log file can be set with the error-log setting.

error-log:      /var/log/mailerq/errors.log

The file must of course be writable for MailerQ.

Send logs

MailerQ creates logfiles for all delivery attempts. The directory in which these log files are stored can be set here, as well as the name of the logfile. Normally, MailerQ appends a number after each logfile, for example "attempts.log.12". MailerQ moves on to the next logfile when the max size or max age of a file is reached. The history option holds the max number of old files to keep on disk before files are removed.

The configuration file holds the following options:

send-log-directory:         /var/log/mailerq
send-log-prefix:            send-attempts
send-log-maxsize:           100MB
send-log-maxage:            3600
send-log-compression:       gzip
send-log-history:           100

The "send-log-directory" is the directory where logfiles with send attempts are stored. The directory must be writable for MailerQ. The "send-log-prefix" is the prefix for the send log. The default is "attempts.log".

The "send-log-maxsize", "send-log-maxage" and "send-log-history" control log file rotation. When the current log file reaches its max age or its max size, MailerQ rotates the logfiles and continues writing to a fresh and new log file. The maxsize setting is required (default is 100MB), the maxage setting is optional. If you leave it out, MailerQ will only rotate the files when the current file reaches it max size. When logfiles are rotated, MailerQ only keeps the "send-log-history" latest log files on disk. All older files are removed.

To reduce disk utilization, you can turn on log file compression with the "send-log-compression" setting. Only gzip compression is supported.

Received messages

Incoming messages - messages sent to the SMTP port of MailerQ - are logged to the received log file. These settings work exactly like the send log settings:

received-log-directory:     /var/log/mailerq
received-log-prefix:        received-messages
received-log-maxsize:       100MB
received-log-maxage:        3600
received-log-compression:   gzip
received-log-history:       100

The log file only holds messages that are received over the SMTP port. Messages dropped in the spool directory or that are injected using the command line interface are not logged.

Download logs

MailerQ can create MIME message itself. To do this, MailerQ sometimes has to download resources from the internet (like images that should be embedded, or attachments that should be included in the mail). All these download operations are written to the download log.

The settings for the download log are essentially the same as the settings for the send-log:

download-log-directory:     /var/log/mailerq
download-log-prefix:        downloads
download-log-maxsize:       100MB 
download-log-maxage:        3600
download-log-compression:   gzip
download-log-history:       100

A word about logging

MailerQ writes all events to RabbitMQ message queues. The recommended way of handling events is therefore to write scripts or applications that process the data from these message queues. This is much more powerful than periodically processing log files:

For all of the above reasons, the first MailerQ versions did not even have logging capabilites. We did not want to slow down our high performance MTA by having it write data to disk - it was up to other scripts and applications to read the results from the message queues, react to them and write data to appropriate log files. With this architecture, hiccups in disk IO could not slow down email deliveries.

However, although this worked (and works!) perfectly for us, we found out that many users still want to have log files to monitor what is going on. So we've added the logging feature, and we use threads to prevent that IO hiccups can be a problem. But keep in mind that once you find yourself writing cron jobs to process log files, you would probably be better of writing a script that processes data from the message queues.