Emails again?

It is extremely unlikely that any e-mail to ever get delivered to the wrong recipient. In some cases, it may get delayed a few hours, possibly even by a few days in some very rare cases.

What CAN appen is that the header of the e-mail can be forged to appear as if it come from someone else. This is always the case with SPAM messages.

There is also the possibility that, for any reason, the intended recipient inbox can't accept the message or that some over agressive SPAM filter could delete a legitimate message.

Finaly, check is you have any filter active that you did not set yourself.
An e-mail filter can be set to automaticaly forward a message according to some rule, then delete the original message.

This is a real possibility if your account have been hacked.

Check ALL settings of your account, taking particular attention for any recovery options and any mail filter. If there is ANY THING that you don't recognise, blank out the entry or put in what YOU want. Change any secret question answer.
Check EVERY THING !!!

Electro
November 2014

What address is (b) seeing on the message?

For a mail message to go astray by accident is very unlikely - I have never heard of it happening.

To make a message deliberately go astray is somewhat difficult and in doing so would give the game away - better by far to just send a duplicate and avoid the risk of discovery.

SMTP Pat
November 2014