NTmail is a handicapped program
When Gordano (the people who wrote NTmail) released NTmail 4 (version 5 is affected as well) they decided to make every one of their customers part of their own private police force.
How you ask? Well, someone somewhere hacked a version of NTmail and put it on the pirate market. Every version of NTmail tacks a header with a text string on to each email identifying the customer and the version and a serial number (there is no way to turn this feature off even though handing out version numbers is a known security risk). When Gordano found out about this pirate version they decided it would be a good idea to block mail from it. So any email that contains a received header with the string "(NTMail 3.03.0014/1.aabv)" in it gets bounced, this string is the identifying mark of the pirate version of NTmail.
So, for ISP's who are running a unix based mail server, say sendmail or qmail, there is no problem, but for ISP's running NTmail version 4 or higher you cannot receive mail from an unlicensed NTmail server.
No problem, right? Wrong, suppose you are an ISP, one of your customers is subscribed to a mailing list. Well if someone from an ISP running the pirate version of NTmail posts to this mailing list then guess what happens? You got it, the email bounces back to the mailing list and YOUR customer gets dropped. No cure, no way for the honest ISP to explain this to their customer because their customer doesn't care about why it doesn't work, it's obviously your fault since it works fine from AOL.
Gordano's response to this? Well they say contact the ISP running the pirate version, except that doesn't work because that ISP can't email you a response since his email is being blocked from your legal NTmail server...
Below is part of the discussion between Gordano and several ISPs.
Response after finding out about it from one ISP
At this time 12/8/99 9am EST, they are still discussing it on their mailing list. If a solution is reached I'll post it here but since there are tons of NTmail 4 and 5 installs out there I figured I better put this notification up here so you don't go an drive yourself nuts trying to figure out why your mail server is bouncing some mailing lists.