[Mailman-Developers] embedded html problem with large lists

[ryan]

No one has been able to answer this on mailman-users for over a week.  
I'm hoping one of you can help diagnose this problem.

On one of my Mailman lists, I send out messages with embedded HTML.  
These messages contain inline images.

I sent out 2 test messages to a pre-configured list.  For the two tests,
the list had only 6 staff members subscribed.  The message looked great,
everyone saw the content with the image correctly aligned in the body of
the message, between the two blocks of text where it belonged.

However, after I added the 6700+ actual email addresses to the SAME list,
and sent out the SAME email, the image was removed from the body of the
message and included as an attachment instead.  This essentially made my
pretty little email look like crap.

Again, the same list (same configuration), and the same email were used
for the successful test, and the unsuccessful real mail-out.  I cannot
stress enough that these were the *exact* same emails, for both the test
with 6 users, and the later mailing with 6700+; the filesizes were the
exact same, and the HTML was unchanged.

So... my only theory is: at some cut-off point, Mailman decides that there
are two many users on a list to embed html/images into every email.  
Perhaps because it is more efficient to store the html once and then
reference it as an attachment when each member is emailed?

Or could this be some behavior of my mailer daemon, Sendmail?

Any thoughts?

Thanks in advance,
Ryan