I keep getting "Mail Delivery Failed" emails for emails I didn't send.
Spammers send out thousands or millions of emails every day, often to randomly generated email addresses hoping to hit on a real live address that they can target with their nonsense.
In an attempt not to be blocked, they will 'SPOOF' the sender's email address. Mail servers can easily block emails coming from a spammer's account, but to try to get around this, they harvest email addresses from websites and lists and send their emails out pretending to be the unsuspecting victims of this fraud.
Spoofing is as simple as just filling out a fake email address in the FROM field when an email is sent. But the mail servers are smart enough to see that this doesn't match up with the ACTUAL mailbox that the mail was sent to, and YOU get the failure note.
There's nothing much that can be done to prevent this, though avoiding publishing your email address on web pages DOES help becuase spammers will pick these up and add to their spoofing list.
If you're getting these, it doesn't necesarily mean that you have a virus, but it would be VERY wise to check that your virus checker is up to date and run a full scan on all your computers and devices - just to be sure.