By default, Postfix tries to deliver mail directly to the
Internet. Depending on your local conditions this may not be possible
or desirable. For example, your system may be turned off outside
office hours, it may be behind a firewall, or it may be connected
via a provider who does not allow direct mail to the Internet. In
those cases you need to configure Postfix to deliver mail indirectly
via a relay host.
Examples (specify only one of the following):
relayhost = (default: direct delivery to Internet)
relayhost = $
mydomain (deliver via local mailhub)
relayhost = [mail.$
mydomain] (deliver via local mailhub)
relayhost = [mail.isp.tld] (deliver via provider mailhub)
The form enclosed with  eliminates DNS MX lookups.
Don't worry if you don't know what that means. Just be sure to
specify the  around the mailhub hostname that your ISP
gave to you, otherwise mail may be mis-delivered.
STANDARD_CONFIGURATION_README file has more hints and tips
for firewalled and/or dial-up networks.