Richard J gave you good advice.

I would add: Don't be so sure your computer is free of malware. Malware
is getting more and more difficult to find, and you often have to try
many different scanners until you're able to identify just what's on
your computer. Sometimes, you never do, especially if it's a rootkit.

But if nothing turns up, it may well be the reply-to problem.

Here are some things I do to minimize my chances of getting caught in a
reply-to situation: I only use my 'real' e-mail address (the one I got
from my ISP) for correspondence with family and friends. For everything
else I use another email address, from one of the many free e-mail
services. If my 'free' inbox fills with spam or gets hijacked I just
forget about it and open another free account. I tell my family and
friends to leave me off any group e-mails - because I don't want my
e-mail address to be forwarded all over cyberspace - unless they're
prepared to hide all recipients' address in Bcc.

---
Ted Zieglar
"Backup is a computer user's best friend."

Richard J. wrote:
If they haven't identified any malware on your computer, then there
probably isn't any, in which case Ted's second possibility is more
likely to be the cause. You may be able to construct some message rules
to delete most of the messages if you can identify common features,
though these are likely also to delete any genuine messages about
non-delivery of mails that you really have sent.