Create an email using Outlook Express|Thunderbird|etc and send it to yourself.
Examine the received email on your unix mail box, before you have downloaded it to you POP account, Use this email as a template to create a script to send a similar email.
The bold|font|color stuff is all html code.
mail is (per some RFCs) intended to be plain-text. The fact that Outlook or similar crap-programs ignore this and send mail in HTML, .doc or whatever fancy format there is (and usually a f*cked-up version of that fancy format, not even a clear version of it), is just that: the violation of a set standard.
There is the MIME standard for such cases, when a mail has several parts attached to it in different formats, but the mail body itself - as per RFC 822 - is intentionally plain-text.
Instead of seeking ways to use tools outside their specification and bend standardized protocols to work in a non-standardized way better find ways to educate your users. If you want headers to display in bold find a way to make your mail program display this way instead of inserting obtrusive code-parts into a meant-to-be plain text document. Change your applications instead of tinkering with the interfaces and protocols in between.