Plait has a built-in email function, which uses Sendgrid to do the actual emailing. So if you want to send emails from Plait you will need to set up a Sendgrid account (which should be free for small providers). You will then need to enter the login details in your Organisation setup.
You can also set up a signature that everyone in the organisation will have added to all their emails. You can use this for your company information, disclaimers etc.