What is Postmaster email?

According to email standards, the local-part "postmaster" to the left of the "@" in an emal address is reserved for use by the person, group, and/or automated processes that manage email at the domain to the right of the "@" sign. Originally, the address corresponded to one or more humans, but these days it is usually automated processes. They are probably writing to tell you about problems delivering mail that you sent. Alternatively, it is possible that somebody has forged the return address to try to make you think that has happened.

You would receive these messages if you wanted to know what problems your outbound mail ran into (addressee unknown, bounced back as spam, unreasonable delay in delivery, etc.). Of course, you might find that the return address was forged, and the essage is something else.