ISPs blacklist email domains from time to time. It's a rather large hammer to solve the problem of spam being generated somewhere in a domain.
Yeah, nemail's domain and servers are listed in multiple RBLs.
I have a feeling it will just work once he gets his servers un-blacklisted.
edit: Also, his reverse resolution for ip -> name doesn't point to his domain, but rather an IP in a dynamic IP address pool. Many sending entities filter on this, either by name (less common since there are valid uses for multiple names for a server, though it will usually add to the "spam score" when processing a message) or by listings of dynamic IP pools.
While these things should affect mostly outgoing mail, many places are increasingly using blacklists on sending mail to those listed servers also in various ways to try to minimize the flow of spam TO various relays, etc.
got a few links for me, please, where i can reproduce your results?
my inbound smtp ip addresses (aka mx records) are pseudo dynamic (at least in the dynamic range) IPs, thats correct. my outgoing mails however are handled through a rented server in a datacenter with proper static ip and PTR record. that ip is not blacklisted as far as i can tell.
never monitored the blacklisting status of my mx record IPs, as i didn't ever hear before of the fact that ISPs may block outgoing mail to blacklisted MXes.
so thanks so far for your analysis but i'm not sure if that isp behaviour (if it is the reason for my issue here) is very smart... i have ordered a business line recently and as soon as I got it installed, at least one of the mx records will be a proper static ip with a proper PTR. i might then remove the second mx for testing.
removing my pseudo-dynamic IPs from all blacklists won't work, I'm afraid, as many blacklists don't allow removing dynamic IPs.
oh, btw i strip the smtp headers on my outgoing mails, so the receiving smtp server only sees the smartmxes proper static IP which has a PTR.
my mailsystem is up and running for years and used by like 20 users without any issues. even communicating with google mail accounts works without any issues...