Unfortunately, my phpbb forum is not able to send mail to XXXXX@yourgooglehostedomain. If members use their XXXXX@yourgooglehostedomain emails, they are not able to receive activation emails from forum. Topic subscription notifications never reach members with XXXXX@yourgooglehostedomain emails either. It is quite annoying.
After searching the forum here, I got this solution, http://www.phpbb.com/community/viewtopi ... 6&t=616835 and it works for me. All emails except email@yourgooglehosteddomain are OK to receive mails from forum.
I guess there must be conflicts between my email server and google for your domain email though I do not know where the conflicts are.
I make a modification on SMTP settings. Just replace your gmail account with your google hosted domain email account and every member can receive email from forum without any problem.
Use SMTP server for e-mail: Yes
SMTP server address: tls://smtp.gmail.com
SMTP server port: 465
Authentication method for SMTP: PLAIN
SMTP username: account@yourgooglehosteddomain
SMTP password: (password)