When setting up services like Mastodon, Peertube, or Wallabag an SMTP service is usually a requirement. The best I have found is Mailgun. Mailgun gives you 10,000 outbound messages for free, and it is easy to set up. To get started sign up here.
Creating a new Domain:
After signing up go to the Domains tab and click Add New Domain.
Next, add your domain.
Once you have created these records the icons next to the records should have a green checkmark instead of the yellow and red icons below. It may take up to 24 hours for the green checkmarks to appear because your DNS records need to propagate around the internet.
Once all your records are set up you can start sending messages to authorized recipients!
To send an email to any recipient you will need to add your credit card. Don't worry you won't be charged unless you exceed the 10,000 message mark for the month, and you can prevent this by setting message and validation limits on the Account settings page. It should look something like below
You are now ready to start sending messages!