This just started last week. On some of the emails the users send out, they get this message back. I've checked all the blacklists and they are not on any of them including the barracudacentral lookup. Can anyone shed some light on this for me?
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<server.domain.local #5.5.0 smtp;553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)>