Customer with Hotmail or Gmail account not receive Order

Hi everyone,

We have this big problem and can't find any solution after a lot hours to read and read in the web..
Our customers registered with Hotmail and Gmail accounts Can't receive the order confirmation mail, But the other topics are fine, Let me explain it:

The customer with Hotmail or Gmail account receive:

Mail after register (✓ YES
Mail order update (✓ YES
Mail password change (✓ YES
Mail directly from our e-mail account through Webmail (✓ YES

.. But

Mail after place a order (x NO

We use Opencart 1.5.5.1

We use the MAIL protocol and test on SMTP ...but not work

Note: We made a test to other mail accounts (Ex: Otherdomains.com) and was receive succefully in all cases. We have Cpanel with Hostgator and after some test they say that all is fine with his service..

How we can fix it..

Thanks!

Is there something in error log?

Could you please share your website link so that we can check after placing an test order.

can be IP of hosting provider is blacklisted by Google and Hotmail. The Hotmail has policy where host providers should register their IP range to allow the email delivery to their service.