/etc/hostname - This file in my Ubuntu server has the following content.

myhostname

/etc/mailname - This file in my Ubuntu server has the following content.

mydomainname.tld

/etc/postfix/main.cf - This file in my Ubuntu server has the following lines, among others...

myorigin = /etc/mailname
myhostname = myhostname.mydomainname.tld
mydestination = localhost, myhostname.mydomainname.tld