We use Verity to extract text from attachments for building our full-text index. When Verity bombs on an attachment, we're doing the wrong thing and rejecting the IMAP APPEND (this is
bug 13250 -- please vote for it!).
The easiest way around this would probably be for you to disable attachment indexing during the
imapsync, then reenable it for normal operation. After the import, just use the admin console or command-line tools to reindex the imported mailboxes.