phpBB3 [version 3.0.3] by default cannot set the display name in the headers of outgoing emails. You could try all you want from the administration control panel but a look under the hood in the source code will tell you its almost impossible.

Fixing it is pretty easy.

open file phpBB3_root_path/includes/functions_messenger.php
find:
$headers[] = 'From: ' . $this->;from;
replace with:
$headers[] = 'From: Display Name ' . $this->;from;
find:
$headers[] = 'Reply-To: ' . $this->;replyto;
replace with:
headers[] = 'Reply-To: Display Name ' . $this->;replyto;
find:
$headers[] = 'Return-Path: <' . $config['board_email'] . '>;';
replace with:
$headers[] = 'Return-Path: Display Name <' . $config['board_email'] . '>;';
find:
$headers[] = 'Sender: <' . $config['board_email'] . '>;';
replace with:
$headers[] = 'Sender: Display Name <' . $config['board_email'] . '>;';

That should do the trick. Did it work for you ?