Returned email which I had never sent

I have a website built with Wordpres/Woocommerce.

I have set up an email address which I get emails to when someone places an order.

I woke up this morning to 2 emails which I was quite excited about, until i read them.

They where headed:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

Reason: delivery retry timeout exceeded

and the other one was the same with a different email address:

Only problem is, I never sent these emails.

when i searched for the top email on google, Avast popped up and warned me that they had blocked a malicious file.

I was wondering if anyone knows what may have caused this?

could my site have been compromised?