Email Address Auto Completion Overrides What Was Wanted
Many email readers have a nifty feature of completing the email address soon after you start typing it in.
Which is great when it gets the right address, it saves you a lot of typing.
Sometimes people don't notice that the wrong address was filled in
Check you Outbox for the addresses the email was sent to, if you have sent a message, but it never arrived.
It is possible to turn this feature off in most email client
Google for "Email Address Auto Completion" along with the name of your email reader.
Email Address Auto Completion Outlook