In the edit screen for your domain at Fastmail, the last item shown is Routing. You can manage the routing with several choices:
- Subdomain routing: If you set this incorrectly you won't get delivery to the proper user based on the subdomain. You normally want to have this set to the first choice.
- Routing: This should be set to internal if your email is delivered to the normal Fastmail servers.
You can also see
Fastmail Help concerning these settings.
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