PHP mailing problem

Hi there,
Just a little problem I couldn't solve lack of time,
I've just redisigned the "L'EXPRESS DE MADAGASCAR"'s webpage
(http://www.lexpressmada.com) with new PLAIN/HTML mailing list .

So I noticed that a lot of people don't accept PLAIN/HTML mail format
so they received the source code instead of the content of the mail.

Can somebody point me out to the easiest solution ?

Thanks in advance
Rija

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