For messages that are synced from a local Maildir to a remote GroupWise IMAP
server, offlineimap adds a X-OfflineIMAP header and a blank line at the start
of the message. This breaks the message as the rest of the headers are then
considered to be part of the content. I think that this problem is not
specific to GroupWise but would happen on other servers that do not support
the uidplus extension.

Here is an example of what a header looks like after synchronization:
"
    X-OfflineIMAP: 1417965986-5747530569

    Date: Thu, 2 Oct 2014 12:50:59 -0700
    From: Benjamin Poirier <bpoirier@suse.de>
    To: [...]

"

I verified that this problem is fixed by the following upstream commit
e8db121 Fix improper header separator for X-OfflineIMAP header