Exchange mail flow not working? Check your (Cisco) firewall!
I’ve come across this issue several times: External mail (or mail between Exchange servers) cannot be delivered, however when you check with telnet the Exchange server(s) are responding. When you check via telnet on the external IP you get something similar: In this case it was a Cisco ASA firewall that had (E)SMTP filtering feature (also called Mailguard) enabled, which is the default setting. Unfortunately, this feature filters very strict and blocks extended commands that are allowed by RFC5321 whichRead more