phpbb has no control over how email is sent. this is all on the server side.
the host decides what type of email sending process it will provide and phpbb provides two possiblities. normal php mail and smtp.
the size of the board is usually not the problem it is how many emails are being sent. that could be thousands with a small board or only a few with a very large board.
basically, if your board is sending out enough emails to strain the server of your hosting company they will let you know and tell you how to solve it.
I am available for custom work on a donation basis. Please send me a PM with your needs.
Premium phpBB 3.2 Styles
OK, so what's the speed of dark?