Your email address has been spoofed. It's common and annoying, but not harmful, and will subside shortly. Spammers often use real email addresses in their 'From' message headers. They don't have to hack you to do this, the same way someone could write your return address on an envelope and mail it. Nothing personal - they choose email addresses at random. When some of their spam email gets rejected by old or inactive mailboxes, you get the return receipt errors. It's called backscatter.
By the time you're getting the backscatter emails, the spammer has already moved on. So, really it's about managing the influx of return receipts, which can be done by a simple 'sort by' select and bulk delete, or by putting in place a filter to automatically delete any further undelivered receipts. Just remember to remove the filter after a day or two so that you're notified about legit undelivered email.
In rare cases, your email account may be compromised and be being used to send spam. We will know if that's happening because the Sent email volume will trigger a security alert. In that case we'll notify you to reset your password and ask you to run a malware scanner just to be sure your machine is clear of any bots (Malwarebytes is a great free one).
Strong passwords help prevent hacking, and it's always good practice to use a strong password for email. Want to know how strong your password is? Try this site: www.howsecureismypassword.net.
We take proactive steps against spoofing on our servers using DKIM (Domain Keys Identified Mail) and SPF (Sender Policy Framework), however these technologies are simply not able to prevent all spoofing. Future security measures will squash this problem, but at current it's just a part of using email - yes, even Gmail has it.
The good news is that this is temporary. Spammers don't dwell anywhere for very long, so spoofing usually only lasts 24 hours or so. They'll quickly move on to another batch of email addresses and carry on their work.
It can be alarming and annoying, but it will subside shortly and is not harmful.