OT: Sendmail related...

Steve Freegard steve.freegard at fsl.com
Wed Jun 24 16:49:08 IST 2009

Christopher Sweeney wrote:
> Alex Neuman van der Hans wrote:
>> Would you like to share? :-)
> Simple; Sendmail will not allow unresolvable domains by default *unless*
> you have set:
> FEATURE(`accept_unresolvable_domains')dnl
> So make sure that it's dnl'd in your .mc file
> The only problem is DNS is not always pefect and you get a lot of false
> rejects especially from smaller ISP's or companies that do internal
> hosting as sometimes a DNS timeout will happen and now sendmail will
> reject the message falsely.  I found it to be more of a pain to do that
> way as I was always getting support emails complaining about why they
> couldn't email someone they had before.

Sorry; but that's rubbish.  Sendmail will tempfail on DNS failures (as
will all other MTAs) and will only reject if DNS returns a NXDOMAIN
result (e.g. domain doesn't exist).

Allowing unresolvable domains at SMTP opens you to serious abuse from
spammers simply 'inventing' domains at will to avoid blacklists etc.
which is why by default MTAs do not allow this.


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