Well once you have some info on the message to search for, search for it in that users /opt/zimbra/store// <== not username

then grep through the whole directory for the message. once you have the correct message (you can 'cat' or 'more' to read it and make sure) use "zmlmtpinject" to put the message back in the users mailbox.

I left out some details but you can find them here, or on the wikis.