Email Authentication Gateway Software? (Challenge/Response)

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# 1  
Old 12-30-2004
Email Authentication Gateway Software? (Challenge/Response)

Happy Holiday Season All,

I once found a software package on the web that was pretty cool. The package was used in conjunction with sendmail or procmail (I think) and would compare the senders email address to a database on the server. If the senders address was not in the database, it would send an email to the sender, requesting they go to a web site (click on a link) and enter a code generated by the server.

After the sender entered the code, the senders address was added to the database and they could send email to the server, bypassing the gate, so to speak. (Challenge/Response)

I searched the web for quite some time and can't find it. I want to install something like that on one of my linux servers (perhaps as a procmail recipe) and am lazy to write it (sound fairly straight forward though).

Anyone know of a utility such as this?


# 2  
Old 12-31-2004
I found this procmail recipe:

How it works

When someone sends you mail from an address from which you've never received mail before, procmail will store their mail away without delivering it to you, and send them a message explaining that their mail hasn't been delivered yet. When they reply to that message as instructed, their original message will be delivered instead, and their address will be added to the list of addresses from which to accept mail in the future.

That's really about it - your friends and correspondents won't be inconvenienced much, and you may never get another piece of spam!
Which also points to:

So I'll hunt around a bit more and install one of the above and test. I might try both.......


PS: See also:

# 3  
Old 12-31-2004
Originally posted by Neo
I found this procmail recipe:

So I'll hunt around a bit more and install one of the above and test. I might try both.......

The .procmailrc above works OK and was easy to install.



# Blocking spam with an accept list, version 030131
# Info at

# Customize all these constants:

# Your e-mail address

# Your name
MY_NAME="Nic Wolff"

# This should be either blank, or a regex that matches any addresses from which
# you get lots of mail that you want to archive but not read:

# This should be a regex that matches all domains from which you know you
# won't get spammed:

# This should be either blank, or a regex that matches the To: headers of any
# mailing lists you're on:

# Customize this to change the autoreply sent to messages from addresses
# that are not in the accept list

Your message has been received, but it hasn't been delivered to me yet. Since I don't have any record of your sending me mail from this address before, I need to verify that you're not a spammer. Please just hit 'Reply' and send this message back to me, and your previous message will be delivered, as will all your future messages.

(If your mail server can run procmail and you'd like to avoid receiving spam, you can find out how at

Thanks, and sorry for the inconvenience -"

# Customize these if they're not right on your system:

# Customize these if you want to put addresses and pending messages elsewhere -
# make sure PENDING_DIR exists!

# Don't change anything else unless you know why you're doing it!


:0 c

# You can mail yourself a request for the list of pending senders
* ^Subject: List senders
:0 fbw


# You can mail yourself to approve an address
:0 fw
* ^Subject: Accept \/.*
* ? echo $MATCH >> $DB
* ? test -e $PENDING_DIR/$MATCH
| ( cat $PENDING_DIR/$MATCH || true )

# You can mail yourself to deny an address
:0 fw
* ^Subject: Deny \/.*
* ? echo $MATCH >> $DENY_DB

# Get sender
:0 hw
FROM=|$FORMAIL -rzxTo:|$TR / _

# If message is from a local bot then archive it
* BOTS ?? (.)
* $ FROM ?? $BOTS

# If message is an NDN then archive it
:0 h

# If message is from a known domain then deliver it

# If message is to a list we're on then deliver it
* LISTS ?? (.)
* $ ^TO_$LISTS

# If message is from a known non-spam UCE sender then dump it
:0 h
* ? $GREP -i ^$FROM $DENY_DB

# Else if message has chicken then register sender and deliver stored messages
:0 Efw
* $ ^Subject:.*Verify.*for.*$MY_EMAIL
* ? echo $FROM >> $DB
* ? test -e $PENDING_DIR/$FROM
| ( ( cat $PENDING_DIR/$FROM && rm $PENDING_DIR/$FROM ) || true )

# Else if sender isn't in DB then send request for chicken and store message
:0 E
* $ ! ^X-Loop: $MY_EMAIL
* ! ? $GREP -i ^$FROM $DB
:0 c
* ? test ! -e $PENDING_DIR/$FROM
| ( $FORMAIL -rt -I"To: $FROM" -A"X-Loop: $MY_EMAIL" -I"Subject: Verify $FROM for $MY_EMAIL"; echo "$AUTOREPLY"; echo; echo "$MY_NAME" ) | $SENDMAIL -t


# Else deliver normally
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