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

I think it's very bad practice (and I mean, really really bad) to ask a user of your website / application to hand over their password for their gmail account. I would never trust any service that requires that. Think about all the things that you could potentially do with such access, you would have access to to the users Google Wallet, can setup a seperate adwords campaign on their account for your own website etc etc etc.

I think the problem you are facing is that (luckily) more and more users turn on 2-step authentication to their accounts.

Why not send email from [email protected] and set the reply-to field to the gmail address of your user? That way you dont need a password, you are not dependend of the SMTP server your client uses etc.

If you realy need to, you should do it trough the Gmail API: https://developers.google.com/gmail/api/guides/?hl=nl
"The Gmail API is a RESTful API that can be used to access Gmail mailboxes and send mail."

Hi diederik,
I completely understand and agree on some parts with you.

but the think is its my users requirement they want to send emails from their own mail address. we already have an option from where users can using our mail system to send emails having their own from name, reply-to address. but we want to add gmail/hotmail integration as per their requirements. we store all users password in hash though. if you have any suggestion to keep it more securely please suggest us.

thanks for the link. i have already seen that link you provided and already started working with it. do you have suggestion to integrate oauth 2.0 with codeigniter?
also just want to be sure that after integrating that way, my users dont need to do anything else from their end.

also another thing, if i use SMTP for google/hotmail, do i must install postfix/sendmail or something similar in my host? 
sorry to ask so many question as im totally new to CI

thanks a lot for your reply once again.