How to User Gmail with the forum software phpBB3?
1) Have the Gmail(google mail) or Goolge Apps's email account works.
2) Log in to your Gmail account, use the menu at the top right edge of the main Gmail window (the one that shows up after you log in), click on Settings, and then Forwarding and POP / IMAP. Under POP Download, click either Enable POP for all Mail, or Enable POP for Mail that Arrives Now (which ever suits your purpose). Configure anything else that appeals to you too. Exit from the Gmail Settings Window after saving your selections (this is important).
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Here are the settings you need to use with your forum phpBB3:
Use SMTP server for e-mail: Yes
SMTP server address: ssl://smtp.gmail.com" onclick="window.open(this.href);return false;
SMTP server port: 465
Authentication method for SMTP: PLAIN
SMTP username: example@gmail.com
SMTP password: Your Password
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