Afterthought about bouncing

Stephe Campbell campbell at CNPAPERS.COM
Fri Feb 13 18:09:33 GMT 2004

Thank you Kai,

I guess what I didn't realize, through all of this, is that I was testing by
turning my catch-all account off, and sending a test to an invalid user at
my domain, and my SMTP server (whose else?) was generating the reply. I was
of the notion that my server would generate a reply for all invalid user
receipts, not the sending SMTP server. Sorry to be so thick.

I assume then that sendmail reacts exactly the same for multiple recipient
emails (CC: and the like) even when they have valid and invalid recipients
at the same domain, regardless of whether the first recipient is valid or

Thanks loads for turning on the light.

Steve Campbell
campbell at
Charleston Newspapers

----- Original Message ----- 
From: "Kai Schaetzl" <maillists at CONACTIVE.COM>
Sent: Friday, February 13, 2004 12:31 PM
Subject: Re: Afterthought about bouncing

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
> 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
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
will try to send the mail back to the sender, if it's a client it will
the user that it was undeliverable. That's how it is supposed to work. If
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
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
and drop the mail. You are not involved in that at all. Look at it like a
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
> *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"
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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