Bob wrote:

There is no read receipt in my sent items - But in Exchange, message
tracking center, I can see that the read receipt was sent by my
account.
Looking through message tracking center, it seems it's only this one
particular email that a read receipt was generated for.

I just can't work out how or why it happened.

You might consider asking in microsoft.public.exchange.admins, since
Exchange seems to be involved. Control of receipts should be possible for
non-Exchange accounts, but the Exchange Admins control receipt request
handling for that type of account. When Exchange and other accounts are
part of the same profile, Exchange has a habit of taking over for many
things, so this might be a side-effect of that.
--
Brian Tillman [MVP-Outlook]