What hostname to put in main.cf for self-hosted postfix, behind HAProxy?

Pfsense (HAproxy as reverse proxy)—->Unraid I run postfix on Debian Bullseye VM (under Unraid) on my home server. It is up and running. I can send the mail out but can’t receive any incoming mail. I’m wondering whether I’ve set a wrong host name or not. At home local network, I can access my Debian server with either debiantest or debiantest.local. When installing Debian, I input hostname “debiantest”, domain “mydomain.com”. My mx record at cloudflare for “mydomain.com” is mail.mydomain.com. In postfix main.cf, I tried specifying hostname as debiantest, debiantest.local, debiantest.mydomain.com. Same results, ie. can receive any mails, but can send mails out. Welcome any suggestion.