Zimbra appends server name to recipient address

[Najtssob]

Our mail addresses are like username@example.com, but sometimes in received mail zimbra changes this to "username@example.com"@mta_server_name, where mta_server_name in the name of our mta server.
So person who receives mail see strangly looking mail address in "To:" field.

If I go to the zimbra mail server or to ironport and see logs everything looks OK and addresses in "to:" filed are fine.
If they were not we probably wouldn't even received this messages.

I gues this is a bug in zimbra's mta because this happens only from time to time. Can you confirm that this is a bug or is something wrong configured ?

[Najtssob]

Did I make myself clear what is the problem ?
Anyone with the same problem or knowing what's happend ?

We are using Zimbra 5.0.18 and zimbra's web based mail client.

Thank you very much.