Mail transfer agent options

https://wiki.debian.org/Debate/DefaultMTA

System mail like cron need an MTA installed on the system, otherwise cron won't send mails: (CRON) info (No MTA installed, discarding output)

Problem: How to configure a simple MTA which

• forwards system/cron mail
• is reachable from containers

There are two options:

1. Install an MTA on the host, make it reachable from containers
2. Deploy an smtp-relay as docker-container. BUT on the host there needs to be another MTA installed that would forward mails to the smtp-relay
3. Ignore host system mail when using a smtp-relay container

Issues with the container approach:

• Cron sends out mails with a non-qualified FROM addr cron doesn't support MAILFROM please support /etc/mailname Possible solutions/workarounds:
  • Use cronie instead of cron, because it allows setting MAIL_FROM BUT cronie is currently only in experimental
  • Wrapper shell like suggested in this comment

dma

dma https://github.com/corecode/dma https://wiki.debian.org/Debate/DefaultMTA/DMA https://salsa.debian.org/debian/dma Slick, no daemon, simple config, queing support

dma issues

Cronmail is rejected and queues up together with bounces see also Make MASQUERADE config setting override -f This is not solved with compiled latest master... Related debian bug report

msmtp-mta

• msmtp-mta
• https://github.com/marlam/msmtp-mirror
• starts a daemon
• Little documentation
• Example config file
• msmtp is configurable via an ~/.msmtprc or system-wide via /etc/msmtprc

Installation:

apt install msmtp-mta
systemctl start msmtpd
systemctl enable msmtpd

issues

Cronmails:

• In order to send mails users need to symlink a system-wide /etc/msmtprc to a user specific ~/.msmtprc !
• Cronmails come in with a FROM: root@FQDN, so the mailhub domain is added, not the sending host domain !

Other MTA options

All these (besides other) packages replace/conflict with the virtual mail-transport-agent package:

Small MTAs useful for smtp-relaying

• opensmtpd https://packages.debian.org/buster/opensmtpd https://www.opensmtpd.org/ https://github.com/OpenSMTPD/OpenSMTPD/

Fully blown MTAs

• exim4-daemon-light / exim4-daemon-heavy complicated config
• postfix

ssmtp

• GitHub

• Debian wiki

• https://tracker.debian.org/pkg/ssmtp

• no daemon, extremly slick, no queue support

• Cronmail will fail like other mails sent simply to root without a fqdn:

  text $ echo hi | mail root smtp_1 | 503 H=($FQDN) [172.17.0.1] F=<root@$FQDN> rejected after DATA: header syntax (unqualified address not permitted: failing address

in "To:" header is: root): unqualified address not permitted: failing address in "To:" header is: root

Unmaintained MTAs

• esmtp-run THIS PROJECT IS NO LONGER BEING MAINTAINED
• nullmailer
  • https://packages.debian.org/buster/nullmailer
  • https://github.com/bruceg/nullmailer, last release 2018-10
  • No documentation, just man pages
  • No daemon, not reachable from containers
  • Queue at /var/spool/nullmailer/
  • Super easy configuration. Example config:
  • /etc/nullmailer/adminaddr: USER
  • /etc/nullmailer/defaultdomain: FQDN
  • /etc/nullmailer/remotes: MAILHOST smtp --port=587 --starttls --user=varac --pass=…