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====== Exim4 - Reject emails with many To/Cc recipients ======

How to prevent emails being sent to huge mailing lists or stopping messages when users have used reply-all against a massive list of users.

**WARNING**:  An email with 1 address in the To header can actually be sent to multiple email addresses if the email system is abused.  For instance a group email address.  Counting the number of email addresses in the To: or Cc: header therefore does not guarantee that you'll block emails that are being sent to many people.  

In the **DATA ACL** (only in the DATA ACL!), you can check the number of email addresses in the To or Cc header by a two step process.  To test these headers, you access it with **$h_To:** and **$h_Cc:**  **NOTE**:  The trailing colon is required. Let's assume that the To: header is:

<code>
"Test user" <john@example.org>,<jsmith@example.com>,"John Smith" <johnsmith@example.net>
</code>


===== Step 1: To header converted to a (colon-separated) list =====

<code bash>
$ exim -be '${addresses:"Test user" <john@example.org>,<jsmith@example.com>,"John Smith" <johnsmith@example.net>}'
john@example.org:jsmith@example.com:johnsmith@example.net
</code>


===== step 2: Now count the number of list items =====


<code>
$ exim -be '${listcount:${addresses:"Test user" <john@example.org>,<jsmith@example.com>,"John Smith" <johnsmith@example.net>}}'
</code>


With current versions of Exim (4.82), what this will ultimately look like in your **DATA ACL** will be something like this:

<file>
deny     message = Too many recipients
         condition = ${if >{ ${listcount:${addresses:$h_To:,$h_Cc:}} }{10} {yes}{no}}
</file>
         
If you are using an older version you can use an older version that iterates over the list and increments a counter:

<file>
deny     message = Too many recipients
         condition = ${if >{ ${reduce {${addresses:$h_To:,$h_Cc:}} {0} {${eval:$value+1}}}} {10} {yes}{no}}
</file>
         
Note that in both cases above, the **{yes}{no}** portion is superfluous.  The return of the **${if...}** statement is always going to be a true or false, so with this in mind, you can leave it off if you want.  I prefer to explicitly put it in because it makes it more readable (to me, and to any who might follow me and have to read this code).