This problem turned out to be due to the following sequence, as intercepted
via a plain unix mailbox:

<meta name=Generator content="Microsoft Word 11 (filtered medium)">
<!--[if !mso]>
<style>
v\:* {behavior:url(#default#VML);}
o\:* {behavior:url(#default#VML);}
w\:* {behavior:url(#default#VML);}
shape {behavior:url(#default#VML);}
</style>
<![endif]-->

As you can see, there is no leading period anywhere. But in reality, there
was one before the 'shape' - it is an HTML class. Outlook sent '.shape' but
did not double the period! As this style snippet happens to be unimportant,
messages display normally if the MTA accepts '.shape' as 'shape' with no
further ado. SMTPL did not do that, as I like for errors to be visible so
that they can be corrected, but as it is Microsoft I have no choice but to
accept that I am wrong and they are right, so I have changed SMTPL to accept
this sequence.

ftp://ftp.lsoft.com/listserv/beta/smtpl.exe

Incidentally, Outlook does double up correctly if the leading period comes
from the message text. It is only automatically generated HTML fodder that
seems to be the problem.

  Eric