[Gllug] Rejecting mail at backup MX

Bruce Richardson itsbruce at uklinux.net
Tue Feb 10 17:56:48 UTC 2004

On Tue, Feb 10, 2004 at 11:31:30AM +0000, Alistair wrote:
> The purpose of the backup MX is to relay to a (hopefully temporarily) 
> unavailable primary.

No, that is not "the purpose" of the backup MX.  The purpose of the
priority field in an MX record is to indicate to remote mail systems
an order of preference for mail systems that they should connect to.
Remote systems are expected to try the lower one first and then work
upwards till they achive success.  There is absolutely nothing in the
RFCs to say what else the mail exchangers should do.

> The users all exist on that primary, they do not exist 
> on the backup, and the backup ought to be making no attempt to process those 
> users 'as if' they were local. 

Bollocks.  Show me the RFC that says this.  I'd be impressed if you
could, because the concept of a backup MX doesn't even exist in the RFCs
and this idea that "backups" shouldn't do X or Y is entirely arbitrary
and made up.

There are many organisations where the "primary" doesn't store mail
either and where all the publicly listed mail exchangers are gateways
that deliver the e-mail on internally.  It's a secure way to do things
and lightens the load on individual boxes.

> Your backup MX is doing its job properly; I would suggest leaving it be.

Please tell me why one mail exchanger should not reject invalid mail
when another one does?  Why impose this arbitrary rule that does no good
but helps spammers and virus writers?


It is impolite to tell a man who is carrying you on his shoulders that
his head smells.
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