When an ISP sets up a mail server, they have to take measures to guarantee the integrity of its mail servers (SMTP servers) as much as possible. To do this, they need some form of identifying who is sending the mail. The two most common ways of doing this is : a.) to restrict relaying rights to its own internal IP addresses (i.o.w. its own user base) or b.) to ask users to authenticate. This is a standard "Best Practice". Its also practice to do a reverse lookup on incoming connections and compare it to a spam list, although this is becoming much less common because its not very effective.
In this case, I see no wrong doing by MTN at all - they are in full compliance with international best practices, and are doing things no differently to how Vodacom and Cell C, amongst others, are doing. They also have no control over how other service providers configure their SMTP servers, which brings me to my next point :
From what I can gather, the SMTP server that MC is using has enforced these best practices, in addition to doing a reverse lookup, hence the reason his connection is getting rejected. A VERY simple solution to his problem would be to use SPA when connecting to the SMTP server of his choice, if that particular service provider has made configuration allowances to offer this service. If they dont allow for SPA, change to a service provider (for sending mail at least) whose SMTP servers do, like Vodacom, MTN, CellC, Mweb, IS, etc etc.
Cheers -
