After migrating from Exchange 2003 to Exchange 2010, we found anonymous relaying to be blocked by default. This impacts Linux systems that aren't authenticating to Active Directory. After some digging, I found that you needed to make a receive connector in the hub transport, and enable anonymous authentication for that. This worked, until the IP address of one of the systems (Linux, not Exchange) changed. Now I am getting:
SMTP error from remote mail server after RCPT TO:<firstname.lastname@example.org>:
host mail.localdomain.com [10.100.50.241]: 550 5.7.1 Unable to relay
(hostnames changed in the message shown here - mail.localdomain.com is my hub transport server for Exchange, and email@example.com is the destination email address)
I checked the receive connector in "Server Configuration / Hub Transport" and it is updated to the correct new IP and has anonymous authentication, bound to port 25.
I also checked "Organization Configuration / Hub Transport / Global Settings / Transport Settings / Message Delivery" and the IP of the Linux system is in the allowed mail server list.
What am I missing?
This is running Exchange 2010 SP1.