Hi !!
A DNS lookup is UDP packet with no requirement of a conversation so its
quick and low overhead.
yes, but it needs big changes to dns server software and the
configuration of that software must be in sync with the mta
configuration. Altough this is feasible it adds extra complexity
to the whole system, maybe it will be better to add an entire new
udp based protocol than trying to get all dns software vendors
to change their software to support ses. Implementing this check
at mta level reduces the amount of changes need for this to run
and the extra overhead need for smtp sessions could be acceptable
as this will only happen when the email is forwarded.
Setting up an SMTP connection using TCP and using it to verify an address is
not only expensive but opens a great way to trawl for addresses.
This is primarily why any good system admin disables VRFY and EXPN on any
SMTP server they use.
I think this is false, addresses could be verified by other means, i.e
trying to deliver mail to that address (more mta's can use this callout
technique to verify the existence of an address). Callouts waste
more bandwidth than VRFY, so disabling VRFY you only force other
people to use callouts, which wastes more bandwith and more resources.
In any case there is a lot of people doing callout verifications for
every incoming email (we also do that) , and with some caching this
does not represent a great inconvenient.
--
Best regards ...
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David Saez Padros http://www.ols.es
On-Line Services 2000 S.L. e-mail david(_at_)ols(_dot_)es
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