Had a problem where a user who forwarded mail wasn’t receiving any. Turns out that port 25 on the machine the mail was going to was blocked.

Edited /etc/mail/sendmail.mc

Was: DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA’)dnl
Changed to: DAEMON_OPTIONS(`Port=smtp, Name=MTA’)dnl

Restart sendmail

If anyone tries to forward their mail to another machine in our cluster, need to make sure that this port is open.