A new advanced option is now available for mail domains.
The Include Subdomains option allows a mail domain include subdomains. By default, mail domains do not include subdomains of the domain name configured in the account control center. When enabled, a subdomain will be rewritten to its parent domain and processed normally. Example: for mail domain "example.com" with subdomains enabled, mail coming in on "sub.example.com" would be rewritten to "example.com". The most specific match will win, so you can still add subdomains to your account with a different configuration than the parent domain with Include Subdomains.