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RABiD BUNNY FEVER
K.T.K

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Useful Exim Scripts
For fighting spam

In the course of my Linux administrative duties (on a cPanel server), I have created multiple scripts to help us out with Exim, our mail transfer agent. These are mostly used to help us fight spam, and determine who is spamming when it occurs.



This monitors the number of emails in the queue, and sends ours admins an email when a limit (1000) is reached. It would need to be run on a schedule (via cron).
#!/bin/bash
export AdminEmailList="ADMIN EMAIL LIST SEPARATED BY COMMAS HERE"
export Num=`/usr/sbin/exim -bpc`
if [ $Num -gt 1000 ]; then
        echo "Too many emails! $Num" | /usr/sbin/sendmail -v "$AdminEmailList"
        #Here might be a good place to delete emails with “undeliverable” strings within them
        #Example (See the 3rd script): exim-delete-messages-with 'A message that you sent could not be delivered'
fi

This deletes any emails in the queue from or to a specified email address (first parameter). If the address is the recipient, the from must be "<>" (root)
#!/bin/bash
exiqgrep -ir $1 -f '<>' | xargs exim -Mrm
exiqgrep -if $1 | xargs exim -Mrm

This deletes any emails in the queue which contain a given string (first parameter)
#!/bin/bash
if [ "$1" == "" ]
then
  echo 'Cannot delete with empty string'
else
  grep -lir "$1" /var/spool/exim/input/ | sed -e 's/^.*\/\([a-zA-Z0-9-]*\)-[DH]$/\1/g' | xargs /usr/sbin/exim -Mrm
fi

Get a count of emails in the queue per sender (sender email address is supplied by sender and can be faked)
#!/bin/bash
exim -bp | grep -oP '<.*?>' | sort | uniq -c | sort -n

Get a count of emails in the queue per account (running this script can take a little while)
#!/bin/bash
exim -bp | grep -Po '(?<= )[-\w]+(?= <)' | xargs -n1 exim -Mvh | grep -ioP '(?<=auth_sender ).*$' | sort | uniq -c

Bonus: Force all non-specified accounts on Exim to use a certain IP address for sending. It would need to be run on a schedule (via cron).
#!/bin/bash
export IPAddress="YOUR ADDRESS HERE"
/usr/bin/perl -i -pe 's/\*:.*/*: '$IPAddress'/g' /etc/mailips
Always make sure your envelope sender is correct
Otherwise things may not work as you expect

When you send an email there may be multiple fields in the email header that specify the email address that it came from and how to reply back to that address. Some of these are:

  • From: This is the field that the user sees in their email client as the "From" address. This field is the most easily (and most often) spoofable as you can put anything you want in this field and it doesn't change how the email is received or responded to. Most systems, in my experience, don't try to protect this field either.
  • Envelope sender: This is used internally by email software to see who the email was really from. Different systems (i.e. spam blockers) can use this field for different purposes.
  • Return path: This field specifies the email address to reply to when you click the "reply" button on your email client.

There can be multiple problems if the latter 2 field are not properly set. Some of these are:
  • Spam blockers may be more likely to identify the email as spam
  • The email might be sent from the wrong IP address. Exim (which cPanel uses by default) might be configured to check /etc/mailips to determine what IP address to send from depending on the domain of the envelope sender.
  • The recipient might reply to the wrong email address when replying to the email.

When sending an email from PHP via the mail function through Exim you can only manually set the "From" header field (of the three) through the "additional_headers" (4th) parameter. This might be possible to remedy on some systems however.

If your server is configured to allow it (it may require privileged user permission), you can pass to the "additional_parameters" (5th) parameter of the mail function the -f Exim option, which sets the envelope sender and return path. For example:

mail('example@gmail.com', 'This is an example', 'Example!', 'From: example@yourdomain.com', '-f example@yourdomain.com');

On a related security note, if you think an email may not be legitimate, don't forget to check the email headers by viewing the original email source. Our servers include many useful headers in emails to help combat fraud including (depending on circumstances) the account the email was sent from, the IP address it was sent from, if it was sent from PHP, and if so, the script it was sent from.