Change the X-OfflineIMAP header to work around possible Exchange MAPI
table overflow problem described in
http://article.gmane.org/gmane.mail.imap.offlineimap.general/1699
(It is unknown whether this problem still exits in current
Exchange versions, but let's assume the worst.)
The X-OfflineIMAP header is neccessary with some IMAP servers to
reliably determine the UID of a new messages uploaded to the server
by using the "UID SEARCH HEADER name string" command. Since this
command compares header name and value it is sufficient to have
a unique header value and a non-unique header name.
Note that a message can have more than one X-OfflineIMAP header if
the message was copied between IMAP folders multiple times.
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>
573d7ed52a