Afterthought about bouncing

Kai Schaetzl maillists at CONACTIVE.COM
Fri Feb 13 17:31:35 GMT 2004

Stephe Campbell wrote on         Fri, 13 Feb 2004 09:50:51 -0500:

> Based on what both of you have said, I may not have to change over. I have
> looked many times for a way to drop messages completely at the MTA based on
> whether the user exists or not, similar to the way a firewall can drop
> packets.

That's exactly what sendmail does by default.

But it looks like at the very least, I will always either generate
> and send a "user unknown message" back to the sender (real or not)

No, you don't. The sender sends either a message directly to your SMTP or to 
another SMTP (f.i. his ISPs). If the user is unknown sendmail will generate a 
user unknown response right at SMTP delivery time, even before the body is 
processed and the other side knows it can't deliver. If it's another server it 
will try to send the mail back to the sender, if it's a client it will inform 
the user that it was undeliverable. That's how it is supposed to work. If you 
don't bounce these messages it means that people sending legimate mail will 
not know that they sent a message to a recipient with a typo and it never got 
to the recipient. There is NO mail your SMTP produces here unless you have 
opted to get a postmaster notify to yourself on any such bounces.
> If I don't use the catch all account, though, am I really returning the
> email back to the sender, or just the notification that the user doesn't
> exist and dropping the original email?

This is up to the remote MTA. F.i. it could decide to just send a notification 
and drop the mail. You are not involved in that at all. Look at it like a ball 
being thrown at a wall with lots of holes. If it is too big to fit in one of 
them it will simply get bounced, you are not involved.

Either way, aren't I spamming the
> innocent returnee who probably never sent the original?

No, it's supposed to work this way. See above - otherwise messages go to a 
black void and no one knows that it didn't reach the recipient.

Unless I misread Mr.
> Kercher below, he indicates that my server will not generate an email to the
> *sender*, but won't it still send the "user_unknown" reply back. How do I
> stop the "user_unknown" reply other than using a catch all account?

You can't, either it is unknown or not.

There are two types of "bounces" (or maybe more). "Good" ones and "evil" ones. 
A "user unknown" bounce is a good one in my taste while bouncing a virus or 
spam (or generating a return message "you sent us a virus") is a bad one, at 
least most people consider it nowadays a bad one for obvious reasons. Do you 
see the difference?



Kai Schätzl, Berlin, Germany
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