Sending mail - SMTP service question

I've recently been surprised how many people have been asking me what their SMTP server is. I guess that it's invalueable to be able to get to speak to customers directly but that's not why I'm writing....

After some investigation I found out that Internet Solutions is the only first tier ISP (in SA at least) that doesn't provide SMTP services (for business adsl users). So if one of their connectivity customers wants to make use of our application (POP3/IMAP/webmail) services, we also need to provide them with SMTP. I have no problem with this although when we sell connectivity, we always bundle it with SMTP service. I suppose it's a little ironic because many connectivity providers bundle POP3. I'm struggling to grapple with the theory behind not providing SMTP with connectivity, mainly because of the additional bandwidth/latency needed to traverse tier one networks. Okay, it is a potential revenue (and job) creation service, but how many others do this? Can anyone explain this to me?