|Your outgoing mail server probably needs authentication & you probably haven't configured this in your account settings. This requirement was introduced to help cut-down on the volume of junk mail being sent from unauthorised sources.|
You need to go into your mail account settings, outgoing server settings, and tick the box for "My outgoing server (SMTP) requires authentication".
Then under that, tick the box for "Use same settings as my incoming server".
Then click 'OK' to save the new account settings.