Net Firewall blocking ports

I have been trying to alleviate a frustrating problem with Net Firewall. One of our mail servers is having trouble delivering email to a particular mail server at another domain. When our server connects to theirs to deliver the email, they try to verify the sender’s email address by logging into our mail server on port 25, and initiating a bogus email to the sender’s address (expecting to get a 250 response that the email address is OK).

Net Firewall shows in the logs the blocked attempt. I don’t understand why, as this appears to be the only mail server that is having this trouble with.
I’ve gone so far as to to put in a rule temporarily to allow everything from their IP address that want wants to connect, and it still fails.

The log indicates that the rule allowing all TCP packets from any IP connecting to our server IP on port 25 are allowed (for both directions). I have set this to be the topmost rule, and we are currently running security level 3. Does anyone have any ideas?