I have an Exchange Server (5.5) which fails to send or receive emails to/from certain domains (too many to list.) When I plough through the event viewer to see what's happening it actually talks to the recipient's mail server but then gets rejected. The mail message will then bounce back saying - Protocol error : Unable to route to sender...??? Does anybody know what this means, & more importantly how to fix it. N.B It's not all domains, just some... thanks in advance.

From what you have said I would test the following: In the Address Space field for the Internet Mail Connector ensure that all SMTP mail is going through this connector and that no quirky mail filter is in place. Usually the notation *.* means all SMTP mail will use the connector in question. Also check and test your DNS configuration. There is a utility on your Exchange CD called RESTEST used for checking domain name resolution problems. The syntax is something like: RESTEST -debug domain I hope this helps.