DNS names

[Jim Reid]

>>>>> "Erik" == Erik Bas <ebas at luijk.nl> writes:

    Erik> Why do MX records have to be defined with every A record... 

They don't! However adding an MX record for each host (not necessarily
the same thing as every A record!) in your net is a Good Thing. If
each host has an MX record, it prevents undelivered mail from broken
software. For instance, some people write CGI scripts and the like and
use the name returned by a reverse lookup for constructing an email
address. "Click here for us to swamp your mailbox with marketing
crap".

If they send mail to the hostname, the chances are the mail will get
queued for a few days before bouncing because the host doesn't have an
MX record or an SMTP listener. When the name has no MX record, mail
systems are supposed to deliver to the name's A record. The chances
are that host doesn't have an SMTP listener, so the mail can't get
delivered. OTOH, if the host has an MX record (say for the local mail
server), the mail can be delivered instead of cluttering the sending
system's mail queue. The mail can then be processed by the local
mailhub. It could reject the mail because it has an incorrect address -
it's sent to a host, not the site's real mail address - or else strip
off the local hostname and attempt local delivery.

There's no requirement in the DNS for this. Or in any mail-related
RFCs AFAIK. However, adding an MX record for each host is a simple way
of keeping the mail flowing in the face of broken software or idiot
users who confuse their computer's name with an email address, etc,
etc. OTOH, adding those MX records just tolerates those mistakes. It
also doesn't give the perpetrators of those errors an incentive to fix
those mistakes.