Thanks for the helping.
I tried both methods (with rebooting the instance), but there is no email is sent. My testing involves the user creation, so that I assume that Mantis would send out confirmation email via the preset Gmail account. But, I found that there is no outgoing emails in SENT folder too.
Is there any other points that I need to take care ? Postfix ? real domain / hostname ?
Is there any way that I can debug it ?
In addition, I checked the DB table mantis_email_table. The content of all those emails are in it.