Basic
Open default mail program, create new message with the TO field already filled out.
<a href="mailto:someone@yoursite.com">Email Us</a>
Adding a subject
Open default mail program, create new message with the TO and SUBJECT field already filled out. Essentially we are adding the parameter subject to the href value.
Spaces in the subject will probably work OK, but to be super-extra sure, you can replace spaces with "%20".
<a href="mailto:someone@yoursite.com?subject=Mail from Our Site">Email Us</a>
Adding CC and BCC
Open default mail program, create new message with the TO, SUBJECT, CC, and BCC field already filled out. Essentially we are adding the parameters cc and bcc to the href value.
Also note that you add multiple values to CC and BCC by comma separating them.
<a href="mailto:someone@yoursite.com?cc=someoneelse@theirsite.com, another@thatsite.com, me@mysite.com&bcc=lastperson@theirsite.com&subject=Big%20News">Email Us</a>
Adding Body Text
Just add the body parameter into the ever-growing list of parameters we are using.
<a href="mailto:someone@yoursite.com?cc=someoneelse@theirsite.com, another@thatsite.com, me@mysite.com&bcc=lastperson@theirsite.com&subject=Big%20News&body=Body-goes-here">Email Us</a>