The last release to officially support MerMail was 5.0. 5.5 went back and forth for a bit, but Nortel finally said they wouldn't support it.
I have MerMail on a 6.0 system, and it works fine, but I just had to back out of a 7.0 upgrade, because the mail station ports just disappeared. It has to do with the loss of support in software for the card density used by the MerMail ports.
I'm interested in finding out if anyone has found a work-around.