Emails being sent to strange addresses

I use Yahoo Mail from Android devices. In my Sent box over the last few weeks I see a handful of completely blank e-mails (no subject and no contents) being sent to very strange addresses such as:

Other addresses include, and

All of these addresses (i.e., the ... in the above) start with the string return- and are followed by a very large random-looking string/number.

What could it be?