XF cannot send emails errors "Unable to send email."

I am getting a lot of this error-log registry on XF errors log:

"Unable to send email"
Zend_Mail_Transport_Exception: Unable to send mail. - library/Zend/Mail/Transport/Sendmail.php:137

Any clue?

Hello Jake!
Thanks for the reference.
One question: The Google App domain with MX records, should be the same domain of XenForo installation? or I can use another domain wich I have already associated with Google Apps?
Yep, mail isn't working. Which server are you using?

Admin CP -> Home -> Options -> Email Options -> Email Transport Method

The default server would be your web host, meaning you need to contact them about PHP's internal mail not working. Or you can specify a SMTP server. Here is a guide for using Google as your SMTP server:

Hello Jake!
-Server guys said sendmail is correctly configured.
-PHP internal email is working for Wordpress, so it is very strange it is not working for Xenforo. Both are configured the same.

Is there anything else I can do to get more information about this issue? any other ideas?

Thank you