By default paid Boundless apps are able to send email via a shared domain. However often it is ideal to send emails via your own domain.
In this post I will show you how to add your own transactional email domain via mailgun.
Add your custom domain to mailgun as described here. Their walkthrough will help you setup dkim and spf for emails for your domain.
Once you have added the domain you will be able to access your smtp credentials for this domain under:
Sending -> Domain Settings -> SMTP Credentials
Using the information found here enter it under Settings -> Email for your app in Boundless.
Where domain is the email domain you setup on mailgun (or another provider)
The username and password is found in mailgun on the above page. The host, port and authentication should be set the same as pictured above to connect to mail servers via a secure connection (port 587 and authentication login, with enable start TLS auto checked)
Once you do this you will see your domain appear after the from @ in all email notification tasks.