We've been having an issue which is still a bit of a mystery to us.
We have a Sophos ES1000 appliance. The problem is with outbound mail.
All our outgoing mail (from 2 Exchange servers) go through this appliance. We got reports from some users that they were unable to send e-mail to some external recipients. The error they were getting was;
Generating server: our.mail.appliance
the@recipient.com
mail.recipient.com #<mail.recipient.com #5.0.0 smtp; 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)> #SMTP#
We tracked the problem down to 2 domains from 2 local companies which are both hosting their e-mail on the same server (belonging to a local ISP). Both companies were complaining that they could send and receive with everyone without issues, except with us.
For us the problem was similar, no issues sending or receiving from anyone else. We initially (months ago) found they were listed on the Backscatter blacklist, however, later they sorted that out and are no longer listed.
The outbound anti-spam policies on the appliance were set to always 'Deliver Immediately' just to see if that helped, but it didn't. Today we tested by delivering mail directly from one of the exchange servers, and that worked, so we finally isolated the problem to our appliance.
Finally, we ended up reverting to send all outgoing mail via the sophos e-mail appliance, but turning off all outbound anti-spam policies altogether. That is working now and e-mails are delivered to the 2 domains that we had issues with.
Where is this rcpthosts list that the error mentions and how could we whitelist some domains we want to send to?
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