Hello Gagan. Thank you creating this plugin, I too started with the official plugin and was running into issues.
I was sending multiple Bcc recipients within the header in an array of values as it is shown in the Codex:
$headers = array('Bcc: [email protected]', 'Bcc: [email protected]');
wp_mail($to, $subject, $message, $headers);
The issue was that your header function was overwriting Bcc assuming they were comma separated, so only the last Bcc was delivered. I created a pull request that fixes this issue
https://github.com/gagan0123/PostmarkApp-Email-Integrator/pull/1