Problem with HTML inside mail

[Christian Lenz]

I can't Send Mails to the Apache Mailinglists because the default format of the 
mail is text/html.
All of my Clients (gmx too) uses text/html as default to Send Mail. Always i 
got the Message Mail Daemon can't Receive Mail because it doesnt accept html. 
In Gmx i can Switch it to plain text. On my Mobile i don't know and i don't 
want it. Whats wrong to use html?