For some time we had quite an interesting and diverse landscape of mail host solutions for Zope and Plone.
With the upcoming release of Zope 2.11 we finally have a nice and feature rich default MailHost implementation in Zope 2 based on the zope.sendmail package. Major kudos to Andreas Jung for doing that work.
Today I have spent some hours to fix the standard mail host used in Plone provided by the SecureMailHost product to become nothing more than a tiny backwards compatibility layer on top of Zope 2's standard MailHost. Hopefully this work will go into Plone 4.0.
At that point we'll have a transaction-safe default mail delivery which you can turn into an asynchronous delivery by nothing more than a switch in the ZMI. And obviously it has SSL/TLS support as well :)
If you are interested in testing this, please get the code from http://svn.plone.org/svn/collective/SecureMailHost/trunk.