User Signup - Password emailed to user is encrypted

I cannot reproduce this all of the time.. which makes this annoying. Sometimes the password emailed to users, after signup, is encrypted. To clarify, when a new user signs up, they are supposed to get an email confirming the signup, which includes their user id and password. Most of the time, the password is unencrypted but occasionally (I don't know the conditions) the password is encrypted instead.