Eu configurei um servidor de postfix e parece enviar e-mails na porta 25, não importando o que eu faça.
/var/log/mail.log
Aug 5 15:49:48 TheServer postfix/master[2225]: daemon started -- version 3.1.0, configuration /etc/postfix
Aug 5 15:49:48 TheServer postfix/qmgr[2227]: BA5881D20592: from=<[email protected]>, size=341, nrcpt=1 (queue active)
Aug 5 15:49:50 TheServer postfix/qmgr[2227]: 8EF0B1D203E8: from=<[email protected]>, size=410, nrcpt=1 (queue active)
Aug 5 15:49:50 TheServer postfix/qmgr[2227]: 4B8C71D205F8: from=<[email protected]>, size=355, nrcpt=1 (queue active)
Aug 5 15:49:50 TheServer postfix/qmgr[2227]: 750711D2049B: from=<[email protected]>, size=410, nrcpt=1 (queue active)
Aug 5 15:49:50 TheServer postfix/qmgr[2227]: DE9491D20598: from=<[email protected]>, size=363, nrcpt=1 (queue active)
Aug 5 15:49:50 TheServer postfix/qmgr[2227]: C42411D20360: from=<[email protected]>, size=387, nrcpt=1 (queue active)
Aug 5 15:49:50 TheServer postfix/smtp[2233]: connect to gmail-smtp-in.l.google.com[2607:f8b0:400d:c00::1b]:25: Network is unreachable
O texto acima foi editado para ocultar o nome do domínio.
O postfix é adicionado ao meu firewall, as portas são abertas.
/etc/postfix/master.cf
# ==========================================================================
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (no) (never) (100)
# ==========================================================================
#smtp inet n - y - - smtpd
#smtp inet n - y - 1 postscreen
#smtpd pass - - y - - smtpd
#dnsblog unix - - y - 0 dnsblog
#tlsproxy unix - - y - 0 tlsproxy
submission inet n - y - - smtpd