Drupal 7 webform module is not sending email properly
I found the solution for the problem, it's really easy by the way. I hope this is helpfull for somebody else.

What we need is go to:

admin/config/system/mailsystem


Then press "New Setting" link, select "Webform" in the combobox, leave the "key" field empty and save configurations.

Then a new option is enable as shown in the image:



Then we only need to select the right class for Webform module and save.

That's all, i think this should work for other modules too.