Our client (AAA.com) doesn't use message labs at all. They deal via email with another company (BBB.com) that does. When John@BBB.com sends an email to Sarah@BBB.com, John@BBB.com will recieve an email bounce back with this error:
Diagnostic information for administrators:
Generating server: server.rbguiweg.com
sarah@aaa.com
cluster4.eu.messagelabs.com #<cluster4.eu.messagelabs.com #5.0.0 smtp; 553-you are trying to use me [server-11.tower-21.messagelabs 553-.com] as a relay, but I have not been configured to let 553-you [1.1.1.1, server.bbb.com] do this. 553-Please visit http://www.symanteccloud.com/troubleshooting for 553-more details about this error message and instructions 553 to resolve this issue. (#5.7.1)> #SMTP
Now this only happens when anyone at BBB.com sends AAA.com an email. BBB.com doesn't have this issue with any other of their clients, and AAA.com don't have any problems recieving emails from any of their other clients. Our client (AAA.com) doens't have any problems sending BBB.com an email.
Now, AAA.com PREVIOUSLY used MessageLabs, so we came to the conclusion that MessageLabs still thinks AAA.com is registered with them, and is trying to route the email to an old server, but we called MessageLabs, who confirmed this sometimes happens, but said the domain is not provisioned with them at all.
The technical people at BBB.com seem to think the problem is on our side, and we think the problem is on BBB.com's side.
Can anyone shed some light into what this error actually means, and what could be causing this to happen?