> ist the option "permit_mx_backup" needless when i use "relay_domains"
> in my main.cf?
> my thought is, that another mailserver cant relay mails for an domain
> that is not listed in "relay_domains".
> so what is the special of "permit_mx_backkup" ?
relay_domains: YOU specify what domains you accept.
permit_mx_backup: RANDOM STRANGERS specify what domains you accept.
In addition, YOU can validate recipients with relay_recipient_maps
so that YOU don't become a backscatter source. There is no equivalent
mechansm for permit_mx_backup, making you a backscatter source.