I think I have a tough one. We have Exchange Server 2003 and are having trouble replying to users from a couple of different domains. Here's the kicker: we only have the problem when the reply is sent to a group of multiple recipients including some from the afore mentioned domains.
The error is the old favorite "There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<mail1.ourdomainname.com #5.5.0 smtp;550 relay not permitted> "
As I said, we only have the problem under those circumstances so I doubt it's the usual communication problem between servers. I can see the MX record for the domains and telnet from our server to theirs with no problem. I have contacted the administrators from both domains and they cannot see the emails coming to their servers at all.
I have tried using a smart host which seemed to fix the problem for a short time for one of the domains but doesn't work anymore.
I set up an Exchange Contact in Active Directory with an email address for one of the domains and deleted him from my user's Contacts. That worked for a while as well but then stopped working.
This happens on more than one workstation as well as from OWA or a Blackberry.
I don't think it's connected but we do have Blackberry Enterprise Server installed on second server.
I need help, please!