Subject: MML with plain
From: alice@localhost
To: bob@localhost

A single plain text without MML markup compiles to a unique text/plain
part.

Multiple plain texts without MML markup compile to a mixed multipart
containing multiple text/plain parts.

<#part>
A plain text part can also be wrapped into a MML part.
<#/part>

<#part type=text/plain>
A plain text part type can be omitted because it defaults to
text/plain.
<#/part>

<#!part type=text/plain>
Finally you can write down MML markup by quoting it using the
exclamation point. This MML part will not be compiled into MIME part.
<#!/part>