Debian Email Server – Why is Mail Stuck in Exim Queue?

debianemail-serverexim

On my server, mail is sometimes delayed for apparently no specific reason.

In this example two mails were generated by systemd-cron about failing cron jobs:

mailq output (domains edited):

 3m  1.9K 1i6W6d-0003UT-Ut <server@example.com>
          www-data@server

 3m  1.8K 1i6W6e-0003Uh-1y <server@example.com>
          root@server

After some minutes or when I issue exim -qf the mail is finally sent and delivered without any issue.

/var/log/exim4/mainlog (Domains/IPs edited):

2019-09-07 10:30:04 1i6W6d-0003UT-Ut <= server@example.com U=nobody P=local S=1914
2019-09-07 10:30:04 1i6W6e-0003Uh-1y <= server@example.com U=nobody P=local S=1875
2019-09-07 10:34:13 Start queue run: pid=13447 -qf
2019-09-07 10:34:14 1i6W6d-0003UT-Ut => me@example.com <www-data@server> R=smarthost T=remote_smtp_smarthost H=smtp.example.com [1.2.3.4] X=TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256 CV=yes DN="…" A=plain C="250 2.0.0 Ok: queued as 268D26139FAA"
2019-09-07 10:34:14 1i6W6d-0003UT-Ut Completed
2019-09-07 10:34:14 1i6W6e-0003Uh-1y => me@example.com <root@server> R=smarthost T=remote_smtp_smarthost H=smtp.example.com [1.2.3.4] X=TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256 CV=yes DN="…" A=plain C="250 2.0.0 Ok: queued as BA7EE6139FAC"
2019-09-07 10:34:14 1i6W6e-0003Uh-1y Completed
2019-09-07 10:34:14 End queue run: pid=13447 -qf

When I sent an email from command line echo 'Test' | mail -s test root, the mail is delivered immediately, without log entries for start/end queue run:

2019-09-07 10:35:21 1i6WBk-0003V9-VR <= server@example.com U=root P=local S=397
2019-09-07 10:35:21 1i6WBk-0003V9-VR => me@example.com <root@server> R=smarthost T=remote_smtp_smarthost H=smtp.example.com [1.2.3.4] X=TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256 CV=yes DN="…" A=plain C="250 2.0.0 Ok: queued as 723116139F77"
2019-09-07 10:35:21 1i6WBk-0003V9-VR Completed

Do you know the reason for the delay? How can I have exim process all mail immediately?

My configuration:

up to date Debian Buster
exim version 4.92-8+deb10u2
both, root and www-data (among others) have aliases to me@example.com (external)
standard Debian exim config (using update-exim4.conf)

/etc/exim4/update-exim4.conf.conf:

dc_eximconfig_configtype='smarthost'
dc_other_hostnames=''
dc_local_interfaces='127.0.0.1 ; ::1'
dc_readhost='server'
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='smtp.example.com::587'
CFILEMODE='644'
dc_use_split_config='true'
dc_hide_mailname='false'
dc_mailname_in_oh='true'
dc_localdelivery='maildir_home'

Best Answer

Apparently, this situation is caused by the combination of exim and systemd as described on the systemd-devel mailing list.

TL;DR: To send a mail from the command line exim forks a process to deliver the mail immediately. However, as the parent process exists, the systemd unit for this cron job terminates, and systemd kills all remaining processes in the control group, including the previously forked process responsible for mail delivery.

To prevent this I found two solutions (apart from fixing exim itself):

Using an SMTP client to send mail via TCP to the local exim daemon, such as swaks(1).
Delaying the end of the systemd unit, e.g., by calling exim -qf (queue flush) after calling mail.

Both solutions are not optimal. The first requires extra software and opens a local tcp connection while the second requires root privileges. For now, I will stick with swaks.

Related Solutions

PHP mail() function not working – looks like exim is rejecting for some reason

Exim is trying to deliver the mail to a remote SMTP host and is failing. This is what the:

R=lookuphost T=remote_smtp defer (-44): SMTP error from remote mail server after RCPT TO:

lines mean. It queues these mails up for retry, and skips through the mailq to see if it should attempt to redeliver any of these mails. It won't attempt this straight away, a predetermined timeout has to be reached. This causes:

routing defer (-51): retry time not reached

Finally, when the max redelivery attempts is reached without success, it tries to deliver a mail to the postmaster (which by default is configured as root). However, the default exim configuration is to never deliver to root, so you get:

**User 0 set for local_delivery transport is on the never_users lis**t

There's two problems here:

Firstly: exim can't deliver to this particular SMTP server. Either the destination SMTP server is broken, or Exim cannot deliver to any SMTP servers. Does exim deliver mail to address on other domains? You can test this on the command line with:

/path/to/exim -v 'email@example.com'
message here
^D ( control D )

Secondly, you don't have a valid way for exim to report failures back to you. Configure your /etc/aliases file to make sure that the postmaster and other accounts are properly aliased to a real address.

Exim4 redirect mail sent to @domain1.example.com to @domain2.example.com

Please use example.com instead of domain2.com, example.com (and .org .net) was specifically created for examples and documentation.

In your /etc/exim4/update-exim4.conf.conf you have configured:

dc_use_split_config='false'

However you are changing something in /etc/exim4/conf.d/router/999_exim4-config_redirect which will be ignored because you are not using split configuration.

Either do:

dc_use_split_config='true'

And reload exim4, you may prefer to change it using:

dpkg-reconfigure exim4-config

Or edit /etc/exim4/exim4.conf.template to add your customisations instead and reload exim4 once done.

Debian exim4 has a feature called hubbed_hosts which can be a convenient way to accomplish what you want without having to add your own routers et al.

So if your want your server to send email destined for domain example.org to domain example.com which has MTAs listening on IPs 192.0.2.1 and 192.0.2.10 you do the following:

Create /etc/exim4/hubbed_hosts and add:

example.org: 192.0.2.1:192.0.2.10
reload exim4

Note, no spaces between the IP addresses and they're separated by a colon ':'

What this does is it will relay email destined for example.org to IPs 192.0.2.1,192.0.2.10 (in my example the IPs of example.com). It therefore will ignore the MX records of that domain. The IP address could be anything, as long as it is configured to receive those emails.

Finally there is address rewriting, see: http://www.exim.org/exim-html-current/doc/html/spec_html/ch31.html Perhaps it has something that will fit your requirements. You could use it in addition to hubbed_hosts.

Best Answer

Related Solutions

PHP mail() function not working – looks like exim is rejecting for some reason

Exim4 redirect mail sent to *@domain1.example.com to *@domain2.example.com

Related Topic

Exim4 redirect mail sent to @domain1.example.com to @domain2.example.com