Olaf Kirch - Linux Network Administrator Guide, Second Edition

If you are unable to locate your problem from the error message Exim generates, you may want to turn on debugging messages. You can do this using the -d flag, optionally followed by a number specifying the level of verbosity (a value of 9 gives maximum information). Exim then displays a report of its operation on the screen, which may give you more hints about what is going wrong.

Exim is still under active development; the version of Exim included in Linux distributions is probably not the latest release. If you need a feature or a bugfix found in a later release, you have to obtain a copy of the source code and compile it yourself. The latest release can be found via Exim's web page at http://www.exim.org.

Linux is one of the many operating systems supported by the Exim source. To compile Exim for Linux, you should edit the src/EDITME file and put the result in a file called Local/Makefile . There are comments in src/EDITME that tell you what the various settings are used for. Then run make. See the Exim manual for detailed information on building Exim from source.

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

which causes messages to be queued if the system load exceeds 4. [117] The system load is a standard Unix measure of the average number of processes that are queued up, waiting to run. The uptime shows load averages taken over the previous 1, 5, and 15 minutes.

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 = *

in the configuration file.

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

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

Individual log files keep a copy of log entries for each message so you can easily inspect them. The same information could have been extracted from the main log file using the exigrep utility:

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

That would take longer, especially on a busy system where the log files can get quite big. The exigrep utility comes into its own when looking for information about more than one message. Its first argument is a regular expression, and it picks out all the log lines concerned with any messages that have at least one log line that matches the expression. Thus it can be used to pick out all messages for one specific address, or all those to or from a specific host.

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.

Here are a few of the more useful options you can set in the configuration file:

message_size_limit

Setting this option limits the size of message that Exim will accept.

return_size_limit

Setting this option limits the amount of an incoming message that Exim will return as part of a bounce message.

deliver_load_max

If the system load exceeds the value given for this option, all mail delivery is suspended, though messages are still accepted.

smtp_accept_max

This is the maximum number of simultaneous incoming SMTP calls Exim is prepared to accept.

log_level

This option controls the amount of material that is written to the log. There are also some options with names beginning with log_ that control the logging of specific information.

Exim splits up mail delivery into three different tasks: routing, directing, and transporting. There are a number of code modules of each type, and each is separately configurable. Usually a number of different routers, directors, and transports are set up in the configuration file.

Routers resolve remote addresses, determining which host the message should be sent to and which transport should be used. In Internet-connected hosts there is often just one router, which does the resolution by looking up the domain in the DNS. Alternatively, there may be one router that handles addresses destined for hosts on a local LAN, and a second to send any other addresses to a single smart host ; for example, an ISP's mail server.

Local addresses are given to the directors, of which there are normally several, to handle aliasing and forwarding as well as identifying local mailboxes. Mailing lists can be handled by aliasing or forwarding directors. If an address gets aliased or forwarded, any generated addresses are handled independently by the routers or directors, as necessary. By far the most common case will be delivery to a mailbox, but messages may also be piped into a command or appended to a file other than the default mailbox.

A transport is responsible for implementing a method of delivery; for example, sending the message over an SMTP connection or adding it to a specific mailbox. Routers and directors select which transport to use for each recipient address. If a transport fails, Exim either generates a bounce message or defers the address for a later retry.

With Exim, you have a lot of freedom in configuring these tasks. For each of them, a number of drivers are available, from which you can choose those you need. You describe them to Exim in different sections of its configuration file. The transports are defined first, followed by the directors, and then the routers. There are no built-in defaults, though Exim is distributed with a default configuration file that covers simple cases. If you want to change Exim's routing policy or modify a transport, it is easiest to start from the default configuration and make changes rather than attempt to set up a complete configuration from scratch.