[harmor19]

You'll need to modify a PHP file because I'm pretty sure it can't be done with a plugin. I'vedone it before but I forget exactly what I've done.
I'll get back to you in a few minutes.

Open admincp/email.php

Find:

PHP Code:

array('$email', '$username', '$userid'),
array($user['email'], $user['username'], $user['userid']), 


Replace with

PHP Code:

array('$email', '$username', '$userid', '$userfield15'),
array($user['email'], $user['username'], $user['userid'], $user['field15']), 


Now you can use $userfield15 in the email.

This is untested.