19.4. Mail Delivery Modes

As noted previously, Exim is able to deliver messages immediately or queue them for later processing. All incoming mail is stored in the input directory below /var/spool/exim. When queueing is not in operation, a delivery process is started for each message as soon as it arrives. Otherwise, it is left on the queue until a queue-runner process picks it up. Queueing can be made unconditional by setting queue_only in the configuration file, or it can be conditional on the 1-minute system load by a setting such as:

queue_only_load = 4

If your host is not permanently connected to the Internet, you may want to turn on queueing for remote addresses, while allowing Exim to perform local deliveries immediately. You can do this by setting:

queue_remote_domains = *

If you turn on any form of queuing, you have to make sure the queues are checked regularly, probably every 10 or 15 minutes. Even without any explicit queueing options, the queues need to be checked for messages that have been deferred because of temporary delivery failures. If you run Exim in daemon mode, you must add the –q15m option on the command line to process the queue every 15 minutes. You can also invoke exim –q from cron at these intervals.

You can display the current mail queue by invoking Exim with the –bp option. Equivalently, you can make mailq a link to Exim, and invoke mailq :

$ mailq
 2h   52K 12EwGE-0005jD-00 <sam@vbrew.com>
        D bob@vbrew.com
          harry@example.net  

This shows a single message from sam@vbrew.com to two recipients sitting in the message queue. It has been successfully delivered to bob@vbrew.com, but has not yet been delivered to harry@example.net, though it has been on the queue for two hours. The size of the message is 52K, and the ID by which Exim identifies this message is 12EwGE-0005jD-00. You can find out why the delivery is not yet complete by looking at the message's individual log file, which is kept in the msglog directory in Exim's spool directory. The –Mvl option is an easy way of doing this:

$ exim –Mvl 12EwGE-0005jD-00
2000-01-30 17:28:13 example.net [192.168.8.2]: Connection timed out
2000-01-30 17:28:13 harry@example.net: remote_smtp transport deferred: 
  Connection timed out

$ exigrep 12EwGE-0005jD-00 /var/log/exim/exim_mainlog

You can keep a general watch on what a running Exim is doing by running tail on its main log file. Another way of doing this is to run the eximon utility that comes with Exim. This is an X11 application that puts up a scrolling display of the main log, and also shows a list of messages that are awaiting delivery, as well as some stripcharts about delivery activity.