Backup mail server?

[Paul Howarth]

Andy Green wrote:
> Matthew Miller wrote:
> | On Tue, May 17, 2005 at 08:43:58AM +0100, Paul Howarth wrote:
> |
> |>I think that what he meant was not outbound SMTP, but that if a sole MX
> |>goes down for more than a few hours, anyone trying to send mail to the
> |>domain(s) served by this server would get "Warning: Message Delayed"
> |>type messages from their own SMTP relays, and it was this that he wanted
> |>to avoid, as it might lead his own clients (people, not MUAs) to think
> |>that his service was unreliable when they discovered this.
> |
> |
> | In which case a second mail server with a lower MX priority is a great
> | solution, given the caveat that it needs to be decently maintained and so
> | on.

Where "decently maintained" includes "has list of valid users on system" 
  and equal or more strict anti-spam defences than the "main" mail 
server, as these are necessary to minimise backscatter generation and 
spam injection through the backup.

> I agree with the "don't bother with backup server" idea, but maybe
> something that would make everyone happy is the MX TTL set to a couple
> of minutes, and a backup server that is only placed into the DNS record
> as the MX, *and only accepts incoming port 25* when the primary is down.
> ~ When the primary is back up again the backup machine is removed from
> the DNS record and rejects incoming port 25.

Trouble is, sometimes lack of availability of a mail server is due to 
network issues rather than the server itself being down, so "when the 
primary is down" may only be true from a limited set of places on the 
Net. If you're going to have a backup server, it may as well be up all 
the time. But I wouldn't (and don't) have one.

Paul.