I'm hoping someone here has an idea of where to look next. We have a domain we support which has email sent to it from one particular user bounce with the following message:

Delivery to the following recipient failed permanently:

[email protected]

Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 Unrouteable address (state 14).

We only have reports of bouncing form this one particular user (who is someone we don't support - except they'd like to be able to contact our customer and we're trying to figure out if the problem is on our end). Many people can successfully send to this domain and the user who is getting bounce messages can send to other domains that we support (so it's clearly something specific with the princetonscoop.com domain and not our setup in general). I've reviewed the MX records multiple times and the server logs don't show a connection which generates this error (in fact this error is not one that our mail server would ever return). So it appears that google is contacting a different mail server for some reason. I have tested sending from my gmail account and that works. I believe the sender is using a google-apps account (the account they are using is from their own domain, not a gmail account).

Any ideas on what might be happening here or what to test/investigate next? Thanks.