We have our website with the main domain: domain.com let’s put the forum as a subdomain: forum.mydomain.com
We will use mailgun to send email SMTP.
What recommendation? use main domain or forum subdomain for emails? noreply@forum.domain.com or noreply@domain.com
The default is to use the subdomain. If you don’t want to use Straightforward direct-delivery incoming mail you can change the notification email address in the settings or the app.yml file.