Hello!
Please don't take this post as a veiled request for fundamental changes at a late date - it's really and genuinely a request for information!
When building servers for clients who for one reason or another don't wish to use SME, I usually use Exim as the MTA. I've found it very flexible, stable, and easy to configure - much more so than Qmail or Postfix. Is there any particular reason for not choosing Exim as the MTA?
Regards,
Dave