“Why do e-mails from my webpage come from < username > @ < host## >”

I got this answer from my ISP, but I have no ideas where to change the code??

This is because our servers require you (or your script) to use a properly formatted, valid From: header in your email. If the From: header is not formatted correctly, empty or invalid our system will change the from address to be <username>@<host##>.hostmonster.com. To stop this, you must change the script you are using to correctly use a valid From header.

Examples of headers that should work would be:
From: user@domain.com
From: <user@domain.com>
From: “name” <user@domain.com>

Examples of headers that will NOT work:
From: “user@domain.com”
From: user @ domain.com