I have this odd situation and need assistance:

1) Manually creating an email internal to my company and sent to someone at internet domain A gets delivered.

2) A manual reply to an email originating from internet domain A gets delivered.

3) When a rule is setup to reply to an e-mail originating from internet domain A, it gets bounced back. Of interest is the error message : "Response: 550 5.1.8 sender domain must resolve"

4) Some users from internet domain A are set up in GW as external entities within our GW system, not all.

5) I googled the error message and the first hit is of interest: XWALL & ESAT Forums • View topic - Don't know why domain blocked?. Oddly enough I believe they use Xwall as their spam filtering solution

6) We do use a third party as a spam filtering solution.

Can anyone shed some light on this?