This settings plug-in enable you to send email thanks to gmail service.
Email are send via Gmail service.
So you need to provide a valid Gmail account
Notice that password is stored encrypted and only submitted through encrypted communication to official Gmail service during email submission (like any email client).
Login/password need to be set only one time in a plugin.
To send email to multiple recipient used a comma between each of them like: email@example.com,firstname.lastname@example.org