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

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.

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.

The only way the above can work is when you store an unsalted(!) MD5 hash of the password. A MD5 hash could be used to authenticate yourself to a SMTP server if they support that AUTHTYPE. But Gmail does not support this, they require a PLAIN password and secure it through SSL.
Besides that, the CI mailer does not support any other AUTHTYPE than PLAIN.

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...