What DNS records are needed to ensure pop3 delivery from external mail senders succeeds?

**My setup:** ---------------- * **Network:** Low spec vps server, * **Mail server**: postfix + dovecot + mysql, * **OS**: ubuntu 16.04 (to be upgraded once mail config is complete). I have a single domain (call it mydomain1.com) successfully configured for sending/receiving email. I achieved this by following one of the many guides available through an internet search. Now I have a second domain (call it mydomain2.com) I'd like to include on the same VPS server (same IP address), and the same email service. I'm currently working out how to configure this 2nd domain within the existing setup, from DNS settings to postfix, dovecot, mysql and virtual mailbox OS-level folder structure. What DNS records are needed to ensure POP3 delivery from external mail senders succeeds? Currently: * mydomain1.com is working (receiving and sending) * mydomain2.com is not working (sending but not receiving) * Only email sent from mydomain1.com is arriving at mydomain2.com addresses. * The 'returned to sender' error msg says:

: host C.secondary-mx.nl[00.000.00.00] said: 550 Unknown
    user. Unrouteable address (in reply to RCPT TO command)