Hello Benoit
On 24.05.2012 16:55, Benoit Panizzon wrote:
We have business customers with an own mailservers asking us to provide a backup MX for their mailserver. Usualy we deny such request, because such a backup MX would bounce all spam which cannot be relayed, and anyway, the sending server usualy queues the email usualy about the same amount of time a backup mx would queue it. So we see not advantage, but a big disatvantage.
I do not speak from an ISP point of view, but I hope that my input may be helpful too.
I would only run a backup MX for customers (or anybody else), if the master MX does not reject any e-mails from the backup MX at the SMTP communication level.
And it should also be possible for the backup MX to know all valid users which the master MX will accept e-mail for. Postfix does support "Recipient address verification" [1] (see about 1/3 down the page), even with saving the results locally. An other option is, if the customer is somehow providing regular updates to the list of valid recipients.
[1] http://www.postfix.org/ADDRESS_VERIFICATION_README.html
bye Fabian