This thread has been around for a long time.  I'm having the test mail failure problem despite following all of the guidance here.  I have an older DNS323 on which the email notifications work perfectly but on my DNS345, I've 
never been able to get a status email.  I initially duplicated the 323's info into the 345 and that didn't work.  I have created the appropriate app password on gmail, to no avail.
DNS345 firmware 1.04
User Name: 
[email protected]Password: as supplied by Google
Port: 587
SMTP Server: smtp.gmail.com
Sender email: 
[email protected]Receiver email: 
[email protected]Connection: STARTTLS
Any more guidance?
Edit 1: Never mind.  I decided to alter the Port to 465 and the Connection to SSL, and it now works.  For posterity.