If your mail service is configured to send from the domain you use, it’s fine. You can change the address that Discourse sends from by changing the notification email setting.
For the chicken-egg case where you need to receive email to gain access to your Discourse installation, you can edit the bottom of app.yml that looks like this:
run:
- exec: echo "Beginning of custom commands"
## If you want to set the 'From' email address for your first registration, uncomment and change:
## After getting the first signup email, re-comment the line. It only needs to run once.
#- exec: rails r "SiteSetting.notification_email='info@unconfigured.discourse.org'"
- exec: echo "End of custom commands"