Required Fields for Comments Auto Fill In

Hi,

What I am trying to do is make my comment box just have one user controlled field: comment_author

I then want the comment.php file to take that comment_author field, and have the comment_author_email automatically generated using the format comment_author@mydomainname.com

Ex: If they type in their name as JOESMITH, I want the "comment-author-email" field to automatically be filled in as JOESMITH@mydomainname.com. I don't want that to be a visible, editable field.

I hope this makes sense. I can usually fudge my way through editing, but not this time.

Thanks!