I have the latest version of PHPBB, and use SMTP as my hosting company has disabled php mail, they also use Barracuda anti-spam checker.
More and more hosting companies are tightening up on anti-spam measures ,and the one that has tripped me up is 'Unnecessary base64 encoding' of email headers.
As far as I can tell phpBB always encodes the following headers in Base64, To, Subject, From and Reply-To.
This is triggering Spam Assassin and Barracuda anti-spam software (maybe more but I haven't come across them yet).
It wouldn't be difficult for the PHPBB coding team to change it to only encode if it is necessary, other packages do this.