Josip Rodin wrote:
Yes. Just like any other large amount of traffic could be harmful on big domains.
I will be more precise. Answering a rcpt-to is, in my case, around 20 to 30% of the job of the "storage cluster" to deliver a mail (I am not talking about CPU, just disks IOs). If the number of mails sent as from our domains is equivalent to the number of mails we receive and if everybody use sender verify, it would mean we have to increase our IOs capacity by 20 to 30% (I know, there is 2 "if" and it is a very rough figure).
I guess the counter-argument could be - all those services are explicitly created in order to voluntarily serve requests, but nobody volunteered their server to answer sender verification requests. Yet, a sender verification request is nothing but a three-command SMTP conversation. If someone puts an SMTP server online, and connects it via DNS, it's not exactly strange that other people talk to it.
No, a rcpt-to is not intended to verify an email but to deliver an mail. You may use VRFY if you want to 1) verify an email and 2) check if you are allowed to verify... :-)
IMHO, using rcpt-to to verify sender is just like using resume download to do segmented/parallel downloads. It works but you are using the command in an perverted/antisocial way.
François