One of our mail servers can't talk to any of the earthlink MX servers and after two weeks of trying I've got a queue full of undeliverable mail to their subs.
Previous attempts at getting help via email@example.com are unanswered. INOC-DBA has no directory entry for them. This old NANOG post (http://www.merit.edu/mail.archives/nanog/msg01582.html) has a valid phone number but invalid extension. So I hit 0 and got a tech anyway. That tech instructed me to forward on my concerns to firstname.lastname@example.org which I did, but nothing more than an auto-response has come back a week later now (and yes, I followed the instructions in the auto-responder and re-submitted in the requested format).
Everything I've read indicates that if we are being blocked deliberately I should be getting a 550 SMTP error back. We do not even get a SYN/ACK back (nor ICMP unreachable/prohibited, nor RST) so no SMTP errors are generated from their servers. The MX are all pingable and traceable, so its not a routing problem AFAICT.
The mailman server here has paranoid rDNS setup, does not appear on any blacklists, and has never been the subject of any spam complaints. It runs a listserv for a professional legal association which charges members a fee before they are allowed to join the list, so the possibility of unwanted third parties being subscribed is very near zero.
TIA and sorry for the noise. We tried having an Earthlink subscriber work this from the other angle. All she got was "Earthlink has been blocking port 25 for years you should now this by now!"