Just a note: Zimbra doesn’t do read-receipts yet, but when it does it’ll expose it in IMAP using the standard “$MDNSent” flag from RFC 3503. If you’re seeing “receipt-handled”, it’s probably one of your clients. (We’d love to know which one!)

And sorry for the forced migration to Exchange. We’ll be here for you if and when your workplace reconsiders their choice of mail server…