Nefaria.com

Nowadays, more and more ISPs are blocking outbound port 25 (SMTP) for spam prevention or reduction purposes. This should be of concern to sysadmins who have users scattered across multiple ISPs (such as webhosting services) or corporate sysadmins who maintain e-mail for mobile users, for example. The workaround is to use the mail submission agent (MSA) port, 587. Most ISPs do not block outbound traffic for this port. On GNU/Linux, we can use iptables for this task. With a single command, we can configure any inbound traffic, destined for port 587 traffic to be redirected to port 25:

iptables -t nat -I PREROUTING -p tcp --dport 587 -j REDIRECT --to-port 25