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mail service with no mx (was - Re: Computer systems blamed for feeblehurricane response?)

  • From: william(at)
  • Date: Tue Sep 13 19:14:34 2005

On Tue, 13 Sep 2005, Joseph S D Yao wrote:

There is no requirement - even in this century - for MX records.  It is
a Good Idea(tm).  But not a requirement.  Lack of MX records does NOT
mean that you lose the store-and-forward capability of SMTP.  Lack of a
secondary server, while equally not a Good Idea(tm), does NOT mean that
you lose the store-and-forward capability, only that you exercise it
more often.
I don't disagree but it so happens not all mail software is fully RFC2821
compliant - that maybe either by choice or by ignorance of the authors
or simply not reading RFC closely enough. If you ever wonder how bad it
is - try looking at your Received header lines and compare to what RFC2821
says about them. So yes, I'll say it again - there are mail servers that don't respond appropriately when there is no MX record.

Besides what RFC2821 says, it is also well-known that use of 'A' if
there is no 'MX' is feature to support legacy [pre-1990] systems/domains and for individual hosts that don't usually used to receive email (but still have working postmaster address, etc). And every recent manual, book, etc for mail server software says that when setting up *domain*
to receive email MX record must be setup.

Oh, and also ... please consider that some firewalls try to discern
whether the connection on port 25 is from a mail server or from Telnet.
Could you elaborate on how firewall will determine if the connection is
from mail server or from telnet on port 25?

They both will have the same destination TCP port, both will use random source TCP port number, etc. I really don't see how L4 device (like most firewalls are) can do this unless they keep list of known mail servers ip addresses - and with millions of them I don't think anyone
is crazy enough to compile that into their firewall.

William Leibzon
Elan Networks

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