Thanks for reply, well IPN Log I pasted was wrong, as it was showing IPN txn being tested from Sandbox Tool, here is IPN log:
PHP v5.2.17 :: WordPress® v3.5 :: s2Member® v121213
Memory 37.20 MB :: Real Memory 37.50 MB :: Peak Memory 37.27 MB :: Real Peak Memory 37.50 MB
copyrightwebstamp.com/?s2member_paypal_notify=1
User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.11 (KHTML, like Gecko) Chrome/23.0.1271.97 Safari/537.11
array (
's2member_log' =>
array (
0 => 'Unable to verify $_POST vars. This is most likely related to an invalid configuration of s2Member, or a problem with server compatibility.',
1 => 'If you\'re absolutely SURE that your configuration is valid, you may want to run some tests on your server, just to be sure $_POST variables are populated, and that your server is able to connect/communicate with your Payment Gateway over an HTTPS connection.',
2 => 's2Member uses the `WP_Http` class for remote connections; which will try to use `cURL` first, and then fall back on the `FOPEN` method when `cURL` is not available. On a Windows® server, you may have to disable your `cURL` extension; and instead, set `allow_url_fopen = yes` in your php.ini file. The `cURL` extension (usually) does NOT support SSL connections on a Windows® server.',
3 => 'Please see this thread: `http://www.s2member.com/forums/topic/ideal-server-configuration-for-s2member/` for details regarding the ideal server configuration for s2Member.',
4 => 'array (
\'s2member_paypal_notify\' => \'1\',
)',
),
)
And I checked custom variable, it is started with domain name.