I would like to know if there is a feature in Magento where in if a guest checksout and his email address is already in the system, can Magento log him in before checkout?
No, you can checkout as guest with any email as often as you want. This would require a custom module with an observer. But you don't want to log in somebody without any authentication just because he entered an email address. So either you should require the password before actually logging him in or send an email with a confirmation link similar to the one after registration.
so its possible to prompt them to login first? and just associate the cart items to him?– tom_cruzJul 3, 2015 at 0:17
If customer chooses "Guest checkout" that means he doesn't want his details to be stored in your end. I mean he at least doesn't want an account. It would be unlawful to do that. Jul 3, 2015 at 0:32
sorry , I mean if the email address that guest uses is already existing as a customer, he will be prompted to login and use his account then associate the items he has already picked for checkout– tom_cruzJul 3, 2015 at 0:41
Not possible out of the box but you could develop a module that redirects to the login page if the entered email address already exists. But be aware that this is bad for the user experience and will hurt your conversions. Jul 3, 2015 at 6:57