The "Best mx transport" option controls what happens when the local system is listed as the best MX host for a destination. By default, Postfix reports a "mail loops back to myself" error and bounces the message. Specify "local" to pass the mail to the local delivery agent. You can specify any transport that is defined in the master.cf file. This option sets the "best_mx_transport" postfix variable.
The "Fallback relay" option specifies zero or more hosts or domains to hand off mail to if a message destination is not found, or if a destination is unreachable.
By default, mail is bounced when a destination is not found, and delivery is deferred if a destination is unreachable. This option sets the "fallback_relay" postfix variable.
The "Ignore mx lookup error" option controls what happens when a name server fails to respond to an MX lookup request. By default, Postfix defers delivery and tries again after some delay. Specify "yes" to force an A record lookup instead. This option sets the "ignore_mx_lookup_error" postfix variable.
The "Smtp skip 4xx greeting" option controls what happens when an SMTP server greets us with a 4XX status code. By default, Postfix backs off. Specify "yes" to move on the next mail exchanger. This option sets the "smtp_skip_4xx_greeting" postfix variable.
The "Smtp skip quit response" option controls whether the SMTP client waits for the response to the QUIT command. The default is to not wait. This option sets the "smtp_skip_quit_response" postfix variable.