Customize Welcome Email

Hello, when someone first signs up (before they even pay), they get a welcome email

Subject: [blogname] Your username and password
Username: [username]
Password: [password]
[Login URL]

How do I customize this email? I tried multiple plugins, including
But none of them made any difference. How do I edit this email that goes out to all new signups.

In the meantime, I'm using this plugin to send out an additional email, but it doesn't let me include the login password.