Putmail is a very lightweight MTA or SMTP client, distributed under the terms of the X Consortium License. Its main features are:

Command-line compatibility with sendmail.
TLS support. SSL can also be used with an external tool like stunnel.
Optional per-address configuration. You can use different settings depending on the From: header.
Multiplatform. It works on any platform Python works.
Simple to configure.
Short and sweet. Only 350 lines of code.