Mail server set up for inbound mail on a subdomain to trigger php script

I'm trying to set my company server up to be able to receive emails to a certain account on a subdomain of our main domain i.e.: [email protected]

Those incoming mailers should trigger a php script that will handle some content processing.

I'm running Ubuntu Server 20.0.4, Postfix is installed, Dovecot is installed, I also tried using courier so that's there too, but I couldn't get that going

I've gotten this far:

• I created a linux user and configured postfix to use local accounts as valid mailboxes
• I configured postfix to accept incoming mail at subdomain.domain.com
• I added the A and MX records for the subdomain to properly route in google domains
• I configured SSL on the subdomain (confirmed working properly in ssllabs and through apache when navigating to said subdomain)
• I added some additional configuration to /etc/postfix/master.cf for triggering the php script (right now just a test script/placeholder):

    smtp      inet  n       -       -       -       -       smtpd
        -o content_filter=myhook:dummy
    ...
    # PHP SCRIPT HOOK
        myhook unix - n n - - pipe
          flags=F user=www-data argv=/path/to/script/MailParser.php ${sender} ${size} ${recipient}   

The MailParser.php script just triggers a write to a log file saying "Successfully got the incoming mail at timestamp"

If I send a mail through smtper.net with these settings

• HOST subdomain.domain.com
• PORT 25
• Use Secured Connection, checked
• Email From: [email protected]
• Email To: [email protected]

It successfully triggers the script to write to the log and I can see the "test" message from smtper.net in the user's system mail, so all seems good there.

If I try to send the mail through a normal email client such as my gmail account however, I get a bounce back undeliverable with error:

550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. Learn more at https://support.google.com/mail/?p=NoSuchUser f16si1842833plj.1 - gsmtp

I'm stuck at this point.

The issue turned out to be with the MX records as @anx pointed out. Additionally the SSL Certificate on the domain was not fully configured so that was also causing issues. Otherwise my local server configurations were set up correctly.