Thanks for your help.
The latest has been that I got rid off any CSS internal as well, because a Hotmail receiver wouldn't see the CSS. Hotmail, I believe, bumps the sender's CSS and substitues it with their own CSS.
I also got rid off DIV: they do not display correctly on HTML e-mails.
Right now the HTML e-mail has only Tables.
Also, when I receive the HTML messages that I sent, they display differently in Hypermart.net, Yahoo and in Hotmail. One doesn't display the background color, the other doesn't display a table color that I had, yet, displays the background color, and so on.
Also, sending the HTML code from Outlook and Yahoo is a problem. When I paste the code in the body of the e-mail, it apparently adds some invisible code to the code I paste. I tried turning off Rich Text editor on Yahoo (so no additional code is added to the code I pasted) but the e-mails do not get sent correctly.
So far the e-mail gets sent correctly only on Hotmail, with Rich text editor Off. For the official letter that I'm doing, I, also, need to start using Hypermart.net, and tests haven't been going well there as well (with their Rich text editor Off).
It's all starting to get too confusing; but at the end it's a lot of trial and error.