Re: People forging their From: addresses

[Gunnar Lindberg <]

If there exists a "correct" method I think it goes like:

    1)	If DNS really returns "NonExistant Domain"
	you MAY return 5xx.
    
    2)	Otherwise you MUST return 4xx.

    This isn't implemented in any MTA to my knowledge so I use
    4xx all the time.

Everything else is prone to errors and will, at time, refuse to
receive legitimate mail from legitimate sources with DNS problems.

Please keep in mind that 5xx is *Fatal Error*, i.e. that pice of
mail will be returned - now that's no concern if it was "real" spam
(and if it was really a non existant domain it can't even be returned)
but it can be extremely bad if it's legitimate mail - maybe bad for
your own business (or for your customers' business, or both).

The cost of 4xx is large log files - in my opinion that's cheap
compared to legitimate mail being refused and returned.

	Gunnar Lindberg