It doesn't waste their resources, because it improves the level of
service that they see. Giving someone immediate feedback that their
address is invalid is far preferable to trying to send them a bounced
mail message that will never arrive; and keeping our mail servers
from being bogged down with bounces that will never get delivered
helps free them up to deliver legitimate traffic.
...
That is the same sort of reasoning that spammers use to justify
their activities.
it's also the same sort of reasoning that you used to justify
sending this mail to the IETF list.
- bounces would not bog down your mail servers, since you can discard
or otherwise deal with bounces with no more "bogging" than you are
spending poking at other people's systems.
for a variety of reasons, that's simply not true for the cases where
I'm using it.
(Many spammers want to receive bounces to clean their
lists, because some big ISPs and others automatically black-list
SMTP clients after they've sent to too many bad addresses.)
perhaps, but even the spammers who clean their recipient lists don't
necessarily use valid return addresses, and it's the return addresses
that I'm checking.
- a mechanism that could really determine that addresses are valid
could be useful in a web page to provide immediate feedback to
users. The reality of MX servers, firewalls, sendmail "catch-all"
maps, and other things make the RCPT command too unreliable to use
for that purpose.
BS. a simple syntax check isn't perfect either, but it's useful to
catch some errors. this catches some more errors, without exhibiting
false positives. it also has a cost, which is why I haven't
recommended it for general use. but that doesn't mean it is useless.
Keith