I have 3 different hosts that continually try to connect to my Linode's SMTP server. About every 5 minutes I have a new connect from, disconnect from, or lost connection from message popup in my mail log file.

I've contacted two of the 3 ISPs who control the hosts, but of course have gotten no response thus far. I'd prefer to resolve the issue and not just block the IPs.

Anyone have any ideas on how to get rid of this sort of thing? I've only had a Linode account for a couple weeks, so I'm wondering if these hosts are just trying to contact whoever had my IP before.

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.