It seems I have a virus, I keep getting "mail returned to sender" & when I scroll down, there is a lengthy amount of complete "gobbledygook";- letters / numerals, all mixed up,& the returner, [here it's the Postmaster General] usually states that the message was "probably generated by a virus"
The email address that it says it is returned from, I don't recognise at all, it is different each time. It has just started happening, just as I seem to have got rid of the so-called "sex aid" ads!!!
By my description, do you recognise it, is it a known virus?

I have AV installed.

The email you received was probably returned to you because someone spoofed yer address- put your addy in the 'from' field and attached a virus to the email. Some servers will block known virus attachments and send an email to the sender notifying them that they may have a virus. I get 10-20 of these a day lately.

The other option is that you indeed have a virus so why dont you update your AV software and run a complete scan.

Actually I've seen this problem, where that email is actually the virus. So be careful if it has an attachement of the supposed document you sent. This has been going around lately.