Why does the mail server receive letters from the site, but does not receive messages from regular mail?

Question

Plesk

Those messages that are sent by the site itself (callback and other forms on the site) come to the mail server. However, if I write from regular mail (@gmail.com) to domain mail (@domain), messages do not come at all .
Data: I use the Plesk
control panel . I have VDS . IONOS provider . open ports - 22, 25, 80, 110, 443, 465, 8443, 8447 I connect via POP3 without problems. By SMTP too, but gmail sends a code for authentication to the site's mail, but the code does not come. I use Postfix and Dovecot How to solve this problem? Additional (optional) questions:



How to diagnose what is the reason?
How to test (using ssh) whether messages get through, what prevents them from reaching?