. . . any ISP that PMMail has any problems with (avail OS/2, Windows), but I don't remember if I've ever attached it to MSN.
I do have one client with Exchange Server for their internal messaging and external e-mail. Problem here is that Exchange can send mail to an ISP using SMTP, but it insists on a cranky version of SMTP to fetch mail from the ISP. Most ISPs just have POP mailboxes and even if they do have SMTP available it often doesn't work with Exchange.
The usual solution is to attach to a specialized gateway ISP, which this client did, and the cost kept going up, then they stopped supporting dial-up (IDSL became available to my client just in time), then they decided not to do gateway anymore.
What I did was put Pythaes MailGate on the same server with Exchange. It fetches the mail and distributes it (from one pooled mailbox, from individual mailboxes, or from different ISPs, no problem) through Exchange, and picks up and sends outgoing mail to the ISP. This program is slicker 'n snail snot - easy to set up, extremely configurable and simple to administer. Made in Frogland (where snail snot is quite familiar in the kitchen).