When a field of user profile(page) updated, a email will send to user

[adrianmak]

I added a field to user template, name status.

it has two values, approved and non-approved.

Restricted pages could be reached if that field is approved.

The website is opened for public registration, and the default value of that field is non-approved.

The approval process is manually and it is approved by site administrator

When  a site administrator changed the field value to approved, I want to trigger a email notification to that user to tell him/her, your account has been approved, you can now login or access our member area blablabla.

I want to write a simple module to implement my needs.

But I'm not sure which hook should it focus on  ?

[3fingers]

This module (still a WIP but looks like it's working) could be a good starting point...