Postfix "Unknown Mail Transport Error" Due to Incorrect Relayhost Configuration

Issue

• The Postfix server defers sending emails and displays this error message:

  Jun 17 13:57:41 postfix.lab.local postfix/smtp[14482]: fatal: missing "]" in server description: [
  Jun 17 13:57:42 postfix.lab.local postfix/qmgr[14472]: warning: private/smtp socket: malformed response
  Jun 17 13:57:42 postfix.lab.local postfix/qmgr[14472]: warning: transport smtp failure -- see a previous warning/fatal/panic logfile record for the problem description
  Jun 17 13:57:42 postfix.lab.local postfix/master[14470]: warning: process /usr/libexec/postfix/smtp pid 14482 exit status 1
  Jun 17 13:57:42 postfix.lab.local postfix/master[14470]: warning: /usr/libexec/postfix/smtp: bad command startup -- throttling
  Jun 17 13:57:42 postfix.lab.local postfix/error[14483]: 58DA82079920: to=<user@example.local>, relay=none, delay=1.1, delays=0.06/1/0/0.01, dsn=4.3.0, status=deferred (unknown mail transport error)