I've just installed Zimbra Desktop on Xubuntu 11.10 and configured 2 accounts :

1st Account- POP Account - a POP account (non-Exchange server), and

2nd Account - an IMAP account(Exchange server account configured as IMAP).

No issues with receiving the mails for both accounts.

No issues for sendng mails from POP account. (1st Account)

But when I sent mails from IMAP account (2nd Account), I get NDR with "invalid email addresses" error. However, there is no such problem of sending mails when I send from Outlook Web Access, or Outlook (configured as Exchange Account and also Outlook Anywhere (RPC/HTTP).

Please throw light on where I might've gone wrong.