I had something similar not long ago (not with AOL) and it turned put that mine was an email hack that was sending spam to any email address that I had ever used over a 15 year period. All the faiure messages or returned mail were from the email addresses which were no longer valid so it was 'returning to sender'.
I scanned my laptop thoroughly for viruses and it came up clean so I came to the conclusion that it was a remote hack of my email which was then used to distribute promotions for porn, specialist vaccuum pumps
, viagra and such like.....
All I did (all I could think of doing) was changing my password to something more secure which seems to have done the trick.
Yours may be something altogether different but it does sound similar to what I was experiencing...