What will be the hostname to send email using postfix mail server through port 25?

We know that hostname is saved in /etc/hostname. I have a public domain like mydomain.com. I am trying to send email from local pc installing and configuring postfix mail server through port 25.

Will the hostname be similar to mydomain.com?

What will be the hostname to send email using postfix mail server through port 25?

That depends on how you've configured Postfix.

If you haven't configured it, Debian's Postfix defaults to /etc/mailname, but other versions may have different defaults.