imap_open showing 500 Internal Server Error in php

Question

I am using the following script to access my email from php.

$inbox = imap_open($hostname,$username,$password) or die('Cannot connect to your email account: ' . imap_last_error());

Its working in windows server perfectly.

But in linux server it showing following error

"NetworkError: 500 Internal Server Error".

Any one have solution for this?