Which backup mx method?

Hi,

There seems to be 3 main ways of doing backup mx but I can't find any
comparison between them - what do people here use or recommend?

1) Add permit_mx_backup to smtpd_recipient_restrictions to allow backup
mx for any domains that have MX records added for our host.

2) Add the domain to relay_domains and let dns work out where to send
it on to.

3) Add the domain to relay_domains and add an entry to transport so the
backup mx always sends it to a certain server.


(and i've been told that you should always use relay_recipient_maps as
well as the above).


Thanks

--ian