using google/hotmail/ yahoo api to send mail instead smtp

Dear Diederik,
really appreciate all your helpful responses.

For sending email with CI through SMTP you don't need any special software installed (like postfix/sendmail), thats the job for your hosting provider or your sysadmin. Normally this runs out of the box. Just set the correct config and your good to go.

regarding postfix, im going to use an un-managed VPS. so i might need to install postfix on that. plz correct me if i still dont need that.

You say you store the users password in a hash. Can you elaborate on this? Because if you store this in a secured manner (as a salted hash) it won't do you any good... You should not be able to convert your (salted) hash back to the original password which you can use to send email through SMTP. If your application can convert the 'hash' back to a password then a hacker can do to...
A hash is used for authenticating the user itself, by rehashing the userspassword and compare the hash of the entered password to the stored hash. It's designed as a way of being able to authenticate a user without needing to know the password itself.

so for a misunderstanding. i mixed up user authentication with this gmail password.

I would advise you to pay more attention to security. Either familiarize yourself more or hire in some expert. Making secure software is very hard work, don't take any shortcuts. Sure it will save you some time but consequences can be very extreme...

thanks for the advice. actually im not the developer just the owner of project and i manage the server side only. im just learning some basic so that i could understand that my project is building on a mannered way. 
now im thinking to put all these hassle aside and use SMTP for my users mail sending part. just curious about the server load using this method. for like 5k mail per day would it make any huge load? must add, all these would be transactional/triggered mail, no bulk mails like 100/200 email together.

also i read on this forum that hash is the better way to store authentication password. am i correct? do to suggest anything better?

thanks in advance.