\Forward isn't defined as an IMAP system flag in the spec, so it
shouldn't have a backslash in front of it. Some IMAP servers support
custom flags, called keywords, which aren't preceded by backslashes,
but I don't know if Zimbra is one of them.
It sounds like one of two things may be happening: either your vircom
server uses a non-standard \Forward system flag in violation of the
IMAP spec (which Zimbra chokes on when Larch tries to copy it), or
vircom is using a "Forward" keyword that Larch is somehow mangling
into a system flag (though this seems unlikely). Either way, though,
Larch should do a better job of verifying that the flags it's
transferring are supported on the destination server before trying to
copy them.
If possible, could you run Larch with with -V insane to capture the
full IMAP conversation up until this error occurs, and send it to me
for debugging (after first stripping out your username/password from
the logs)? That will help me track down the root of the problem.
- Ryan