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: return-a1756-676244-676247-ba1aedb9=136622015=8u@dha.biznes1.com

Other addresses include ...@snip.prikn.com, ...@tej.ruhour1.com and ...@gsh.hirareth.com.

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?