The mailing list for listmasters using Sympa

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Html: This mode is used when a subscriber wishes to receive mails sent in 
both HTML and plain text formats only in HTML format.

According to the help file all email should be converted to hml if this 
option is chosen.  However, I have been testing this feature without success 
as I sent both email in ascii text and rich text format with images 
(stationary and signature).  I feel like I am missing something here.  Sympa 
should be able to handle both rich text format and html as what I understand 
from the docs.  Can someone explain this to me or shed some light on this?