Re: [ietf-smtp] DSNs

Sam Varshavchik <mrsam@courier-mta.com> Sun, 26 April 2020 21:02 UTC

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From: Sam Varshavchik <mrsam@courier-mta.com>
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Date: Sun, 26 Apr 2020 17:02:04 -0400
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Subject: Re: [ietf-smtp] DSNs
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John Levine writes:

> With SMTP you have no way to tell who is sending a message that
> arrives from outside, and experience tells us that the majority of
> incoming mail is hostile.  We don't do success notifications or web
> bugs because we don't want to leak info about our users to malicious
> strangers.

Can someone explain something, slowly, to me:

Incoming mail is addressed to a non-existent recipient. Your MTA rejects the  
recipient.

Incoming mail is addressed to a valid mailbox. Your MTA accepts it. As such,  
the sender already knows it is a valid recipient.

I don't follow what information is getting leaked, if a second later a  
success DSN gets sent to the sender. The sender already knows it's a valid  
mailbox, by the virtue of the fact that the mail was accepted.