Luca Cazzaniga wrote:
I saw the description of the variable nonetheless I was asking if is it only useful in case of a backup mail exchange.
if the external IP is not declared as the MX for some domain, you don't care.
if the external IP is the MX for some domain, you may or may not need to care, depending on your setup.
In short - it is not required except in some situations - it costs nothing to add it
