Did you ever solve this? I am having same problem.
I also see this in the mailboxd.log:
2009-04-08 23:20:56,217 INFO [LmtpServer-270] [name=account@zimbraserver.com;mid=6;] mailbox - outofoffice not sent (not direct) mid=284 rcpt='account@zimbraserver.com' dest='address.of.sender.who.sent.to.pop@someserver .com'
I think this is causing the problem:
outofoffice not sent (not direct)
My external test account does not match the
user@domain.com or any aliases for the account I am testing with. I think this may work with my real accounts that fetchmail picks up as they are addressed to a valid name that exists as an alias. Will have to just try and see.