The problem is: if the user is searched by "userPrincipalName" at ActiveDirectoryUnixAuthenticationProvider.java:385 and the users are defined by "sAMAccountName" an Exception will be thrown. So the code bellow line 385 will not be executed! So no search for "sAMAccountName" will be made. Because of this, the E-Mail Plugin won't be able to search for Users/Email-addresses and no notifications will be send.
I hope this bug wasn't mentioned before.