It is a common spammer tactic and it has worked in your case as you have opened it and that could have resulted in more serious issues.
So your actions aren't going to stop these speculative or bounced emails. One likely scenario is that your email address is already on a spam list and those email addresses aren't just used to send spam too, but they are used in the from email address (easily faked). Another scenario is that someone that you have communicated with, friend, colleague, etc. has an infected system and their email address book is harvested for email addresses to send spam to and to use as the from address.
The problem is that dumb email servers and spam filters bounce the email back to the from/reply to email address without thought that this might not be who sent it.
I get these on occasion and I know I haven't sent them (so I know they weren't likely to be undelivered), my anti-spam invariably picks them up or one of my filters will flag it otherwise the mark one eyeball picks them up and they are flagged for deletion. Then my email program is called to download the remainder of legit emails.
What to do, set the avast Mail Shield to High heuristics as that will detect if you are actually sending out multiple emails in a short time frame (generally spam). This could also be an early indication that you may have a hidden/undetected spambot no your system. In most cases you aren't sending out spam, but it is the scenarios outlined above and their bounced email being send back to the from address.