Hi Reuben,
Re: no user interaction. I prefer MAPI for end users as it has no extra configuration required and uses the email client to create the email so users have full control over what is sent via the UI of their MAPI client.
However I'd still on occasions like a user to be able to send via MAPI without user interaction. For example we have a report mail trigger option where users (via a configuration screen) can pick reports from our report list (500+ now!) and request that a given report is mailed automatically whenever it is generated (they enter the to,cc,bcc, subject and message etc for a skeleton email). Once setup whenever they generate a sequence of reports the ones which have a trigger setup are automatically emailed silently to the recipients.
To code around this I now use MAPI for interactive emails and SMTP for the non interactive ones. This works but requires extra work at configuration time as the SMTP option needs a server/port and a from address to work.