HTML keeps changing??!

Hi, when i write certain things in the HTML section on the post/pages area, then click over the visual setting, the html has changed…?

ie if i code my email address, then go to visual, when i go back to the html it has changed to just my e-mail address.

Also when i import html from an ad, then click visual and back to html it has taken a whole heap of code out, making the code useless!

Have i got some sort of auto format thing going on or what?
Starting to get really frustrating, can anyone help me with this. Cheers.