When starting Postfix, you receive an error message in the maillog similar to the following:
Jan 01 HH:MM:SS HOSTNAME postfix/master[#####]: fatal: /etc/postfix/master.cf: line ##: bad hostname or network address: localhost :10026
You encounter the error because Postfix is unable to resolve the localhost.
To resolve the issue, check the /etc/hosts file and make sure that the localhost address is 127.0.0.1.
For example:
127.0.0.1 hostname.localdomain HOSTNAME localhost localhost.localdomain