These warnings are shown only for phpBB emails, not my site's mails. If phpBB could fix this issues to improve the mails reputation, that would be great.-0.105 FROM_EXCESS_BASE64 From: base64 encoded unnecessarily
-0.999 XPRIO A X_PRIORITY header is inserted in your message Please remove the X_PRIORITY header
To test your forum's possibility of email being sent to spam or directly rejected, register a new user with an email provided by Mail Tester and check the results. I've got a score of 7.2 for phpBB emails, and 8.3 for my site's own emails.