ubuntu – Sending mail through postfix with remote embedded applications

I made a local user on the postfix server, installed s-nail, initialized the Maildir structure and successfully sent test mail locally from this postfix server to a gmail address, but we have to send mail through postfix using remote embedded applications. I did not install dovecot etc. as we no need for IMAP/POP3 server (we won’t use any mail client remotely). So mails should be just forwarded to remote recipients sent from remote embedded applications.

Problem is that mails are not beeing delivered (surprise :), can connect to mail server remotely but getting ‘Host or Domain name not found. Name service error for name=localdomain type=A: Host not found’ message (checking with mailq).

Let’s say our domain is stipe.eu, MX is set to the right public IP, SPF record been also set:

mail A <public IP address of postfix server>
@ MX 10 mail.stipe.eu
@ TXT v=spf1 +a +mx ip4:<public IP address of postfix server> ?all

main.cf:

main.cf_1
enter image description here

What settings should i configure so i can send mails from outside? At least with pure SMTP authentication using the local postfix account (local linux user) remotely.

(Ubuntu 20.04, Postfix 3.4.13)

Any idea please? Thank You!