I finally managed to get sendmail working using systemd.
Here is my /etc/systemd/system/sendmail.service:
[Unit]
Description=Sendmail Mail Transport Agent
Requires=clamav-daemon.service spamassassin.service
After=syslog.target network.target clamav-daemon.service spamassassin.service
Conflicts=postfix.service exim.service
[Service]
Type=forking
PIDFile=/run/sendmail/mta/sendmail.pid
Environment=SENDMAIL_OPTS=-q1h
EnvironmentFile=-/etc/default/sendmail
ExecStart=/usr/sbin/sendmail -bd $SENDMAIL_OPTS $SENDMAIL_OPTARG
[Install]
WantedBy=multi-user.target
and my /etc/tmpfiles.d/sendmail.conf file:
d /run/sendmail/ 0755 smmta smmsp
d /run/sendmail/mta/ 0755 smmta smmsp
I am using clamav-milter and spamass-milter, hence the Requires= and After= lines. If you are not using these, probably you should remove those.
Is it wrong to include these dependencies in sendmail.system? The thing is, these milters are not specific to sendmail. Other mailers that support the milter interface can use them as well. And they are not required for sendmail. So I wonder which pakage's responsibility it would be to add these dependencies to sendmail.system or if this is even the correct place to do that.
With the init.d, clamav-milter and spamass-milter install themselves with a lower number than sendmail and always start before whatever mailer is installed. Once you go to explicit dependencies like this, is it clam's and spamassassin's job to know all the possible mailers out there that might use it? Or is it sendmail's job to know all the possible milters out there and state them as dependencies?
Another observation, to get this working, the only way I found to properly test this was to continually reboot. I could get sendmail to start by hand quite early on, but it was not starting by on reboot because of the timing problem in the dependencies. This makes systemd rather more difficult to debug things in my opinion.