Thanks for the note about the flags. As most e-mail accounts in the company were used only with Zimbra’s mail client and Zimbra’s MS-Outlook connector, I assumed all the non-IMAP RFC stuff is Zimbra generated. I don’t remember in which account I’ve seen the receipt-handled flag, but it could be either mine (also used with Evolution) or the CEOs (also used with Blackberry servers).

About the migration – I’m not happy with it but there was so much resistance to a non-MS solution even after the Zimbra trial, that it was impossible to keep the Zimbra server. Most likely it had something to do with our first few attempts at email server which were all F/LOSS based and were all flops to some degree or another, so much that even though Zimbra had delivered, people still wanted their MS fix.