You would have done this on facebook - Facebook asks you to enter your email address and password and it will login to your email send all the people in your email invites.

I guess it uses the URL of the login page, includes the username and password and then uses hotmail's or gmail's compse email page to send emails..

I am not looking for a script or something. I am looking for background informaiton and the skillset i need to develop something similar. I am an average web developer on PHP, ASP, mysql and SQL server. I am not scared of writing perl scripts either.

If you have built something similar, please let me know how I can go about it.
Should I learn "http" or similar for this?

Appreciate your time.