This is the sharepoint log C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\LOGS
I searched th log file to find any related logs of e-mail and found only the followng log
The Incoming E-Mail service has completed a batch. The elapsed time was 00:00:00. The service processed 0 message(s) in total
#160009: The e-mail address 'firstname.lastname@example.org' is unknown
may be there is a problem sending email to external because when I try to create user using internal e-mail everything is OK.
I will try find how to let SharePoint send emails to external