php send mail issue

I am developing a web application (Php+Apache+Postgres on Ubuntu) where registered users can send emails to the admin and vice versa. I have sendmail installed as MTA. The problem is php mail command isnt sending/receiving any mail. The registered users can have any email id e.g. user1@abc.com, user2@xyz.com and admin can be admin开发者_StackOverflow中文版@example.com.

I earlier tried ssmtp MTA but using that if i provide admin's email (e.g. gmail's authentication), then admin can contact users but I could not figure out the other way around communication (from users to admin because it is not necessary that all users have gmail id).

Can you guys throw some light on how to solve this problem such that any user with any email id an contact the admin or vice versa. What would be the best steps to address this problem? Currently I am using a localhost. Thanks for your support.

Edit: The sending of emails is facilitated using a form with to,subject, message fields

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Have you tried switching from sendmail to postfix? Postfix is usually a better alternative to sending mails.

Here's a guide to installing Postfix on Ubuntu.