How to send subdomain emails to main domain?
Regardless of your OS and mail-software, MX-records need to have a priority figure too.
For example, here the priority is set to 11:
@ IN MX 11 mail.example.com # MX-record for the domain itself
* IN MX 11 mail.example.com # MX-record for any hosts or subdomains
These will tell the rest of the Internet, whom to give e-mails sent to your domain (and any of the subdomains).
You will still need to configure the e-mail software on your computer(s) to handle such delivery attempts, but that's another topic.