Unknown hosts connecting to my SMTP

Possibly just some spammers looking for open relays… but in my limited experience, haven't seen them check so frequently. Sometimes I have 5/day.. sometimes none for a whole week. I'd check ordb.org to make sure you're not running an open relay. If you are.. fix it. If not, then I'd block the IP's.

Another possibility is that someone misconfigured you as an MX, or mis-typed an IP address in their own DNS. If there were an easy way, I'd say find out who they're trying to deliver mail to and see if that helps uncover the answer of why they keep trying to connect to you.

Of course, if they're really annoying, you can save some packets (and send a stronger message) by doing a "route add IPaddress reject" for each of them. That is, unless you have a firewall running, in which case you can just stick it in those rules.