Re: Validating email addresses

<rob@dekko.com> wrote in message
OF34246F71.6FAB6DA4-ON05256BA6.0075F211@dekko-1">news:OF34246F71.6FAB6DA4-ON05256BA6.0075F211@dekko-1...
> So how does one verify email, other than bounce back?

You don't.  (keep in mind that some mail systems don't even bounce mail
back, see below)

Used to be that mail systems enabled the VRFY and EXPN directives.  VRFY was
truly wonderful because it would let you just verify that a email recipient
was valid (this is what you really would like to use).  Unfortunately, this
was found to be an exploit of some sort and most mail systems turn it off.
EXPN did something similar, except it would expand an alias to the full
email address (including mailing lists).  It's usually also turned off.  If
it isn't, it should be.

> If I send email to david@midrange.com does it search every domain under
> the sun until it finds one with an acceptable mx record?  If not, then
> which domains does it search for mx records?

Nope, midrange.com has one set of MX's ... You can have any number of MX's
for a domain with differing priorities.  It uses that list of MX's, in order
of priority, to determine where it will send email.  I think there are only
5 priorities (very high, high, medium, low, and very low).  I'm not sure
what rules there are for MX's within the priorities, probably arrival.

Oh yeah, one other thing that I just thought of ... some networks (like AOL)
have load balancing DNS servers that will try and round-robin the servers it
returns in order to spread the load across systems.  I'm pretty sure this
goes for MX records as well as their other servers.  It's been a long time
since I looked at AOL's records, but I think if you ask for the MX, you
might not get the same answer the next.

In midrange.com's case, linux.midrange.com is the high priority MX, with my
UUCP backup as secondary.  John's AS400 should never receive mail for
midrange.com unless the sender is completely broken and assumes that the
main domain name is the mail receiver.  I've seen this happen, but the mail
system was totally hosed (I think it was a cc:Mail system, but what would
you expect?).

On the subject of mail systems that don't bounce delivery errors back, I
don't know if you remember ... but AOL once had a law suit filed against
them for a DOS attack.  Turns out a spammer had been pummeling AOL accounts
and AOL wasn't sending the delivery errors back, but they were holding them.
All of a sudden, without warning, AOL released the held delivery errors (I'm
not sure if it was deliberate or a mistake).  The spammers ISP was totally
demolished because of all the rejected mail messages coming at them.  I
happened to be a remote staffer for AOL at the time and got a real big
chuckle over it.

david