[cdh]

Perhaps I am misunderstanding you, but while it is certainly uncommon, it is absolutely possible to directly deliver mail to a recipient's SMTP server. In many cases, this can be done without any authentication whatsoever.

I run my own mail server. When I send an email via the web interface, would you not agree the MUA is attempting to directly connect to the destination host?

Depending on different factors, it may be classified as spam by the recipient, but from a reputable IP address it shouldn’t normally be rejected outright. Of course there are other factors like SPF, greylisting, etc... but email can be directly delivered.

[eikenberry]

If you send from non-SPF listed IPs you will get rejected in many places and most likely marked as spam in those you don't. SPF particularly and DKIM and other anti-spam measure are in wide spread use and enforced by the receiving SMTP server either directly or via spam scoring system rules.