Small hint from me: If you set up Postfix to relay your mails and your relay host does not have an MX record (anymore), mailq has a lot of mails on standby and you see something like:
Jul 14 12:45:39 myhostname postfix/smtp: 74FBF30501: to=<email@example.com> relay=none, delay=3944, status=deferred (Name service error for name=recip.domain type=MX: Host not found, try again)
And found this FAQ on Posfix.org and you say: "Yeah, the FAQ is right but so what now?". Just boot your VI editor and open
relayhost = smtp.example.com:587
relayhost = [smtp.example.com]:587
so Postfix will not looking for a valid MX record anymore. After this, go to the important work again...