Inbound Emails blocked - 421 service not available

I've got a couple clients, on different accounts, who are having problems receiving mail from specific domains.

I'm not doing any RBL in the exim config, spamassassin isn't currently enabled on the two domains (I realize they should be, I just found that they weren't).

We've got hundreds of other email accounts setup that are working fine, so I can't figure out why these two are being blocked.

I tested the function of the SMTP service with this util, and everything looked fine:
http://www.zoneedit.com/smtp.html

Anything else, besides a transit provider block somewhere on port 25, that could cause this?

Also - I can't seem to telnet on port 25 to the machine either, to test it. I'm on Cox - so this could actually be affecting my home network as well. Could somebody else give it a try?

Telnet 64.62.181.34 25

I keep getting timeouts.