The Median Sib

I keep getting email about emails that have been returned as “undeliverable” - and each time the email in question is from my domain. For example, it might be [email protected] or [email protected] - always some random combination of numbers and/or letters followed by my domain name. Obviously someone is sending out spam supposedly from my domain. I double-checked my account and there is only one email account associated with themediansib.com - mine.

I’m definitely NOT knowledgeable about how this works and how to stop it. Any suggestions? I hate for people to be getting spam and thinking it’s from me. I also hate for spammers to be using my domain’s name. Is this a virus? or what?