Although the thread is three years old, I stumbled upon this thread as I have noticed the same behavior, but only for some accounts. I’ve found the problem in my case and a solution working for me:
Your primary mail address is
givenname.surname@domain.de and you have an alias (in my case not configured on Zimbra) like
surname@domain.de . If your IMAP client is configured with the alias, the recipient listed in the “To” header (and written to the metadata field in the mail_item table) is set to the sender, not to the recipient. If you change the mail address in the IMAP client to the primary mail address known to Zimbra, everything works fine.