How to create a Mailbox in Postfix in /var/spool/mail

Solution 1:

Configuration error

You didn't provide any of your configuration, but the error is quite revealing:

dsn=5.4.6, status=bounced (mail for domain.com loops back to myself)

From this it seems your configuration is missing domain.com either from mydestination or virtual_alias_domains in main.cf, whichever better fits your needs. Please take a look at the Postfix Basic Configuration – with this question especially the chapter What domains to receive mail for.

Additional problems with DNS

Your current (as of Oct 24, 2020) MX records are:

;; ANSWER SECTION:
domain.com.         3399    IN      MX      1 domain.com.
domain.com.         3399    IN      MX      1 mail.domain.com.

However, the mail.domain.com doesn't exists with an A record.