Ubuntu Security :: Postfix - Not Critical - SMTP Server: Errors From Unknown [ip Address] In Local Recipient Table

This is a transcript I get emailed at least once every day, usually about 3 to 10 a day recently.

Transcript of session follows.
SMTP server: errors from unknown[ip address]
<boring stuff snipped>
In: RCPT TO: <server@my domain>
Out: 550 5.1.1 <server@my domain>: Recipient address rejected: User unknown in local recipient table

Session aborted, reason: lost connection Now I cannot seem to find anything via Google, as when I put "server@" anywhere in the string, I just get web hosting or other kroomst. The emails usually come from legit places, usually hotels. Does this mean they are sending bad emails, i.e. they have a Trojan/worm, or is this a live hack attempt?. I believe the later, as I might get upto 3 domains from the one ip address, which is always, NOT associated with the listed domain. Not causing me any issues, except I have been getting a lot recently.