View source for External eMail Warnings

=Customizing Amavis To Add Warning Messages To External Emails= 
{{KB|{{ZC}}|{{ZCS 8.8}}|{{ZCS 9.0}}|}}

In this article you can learn how to add an external email warning message in Zimbra when receiving an email from an external domain. Tested on Zimbra 9 patch 19. The solution is by way of a Sieve filter, it can be enabled per account,cos,domain,server.

Create <code>/tmp/myfilters</code>

<pre>  su - zimbra
  nano /tmp/myfilters</pre>
With the following contents:

<pre>require [&quot;fileinto&quot;, &quot;reject&quot;, &quot;tag&quot;, &quot;flag&quot;, &quot;editheader&quot;, &quot;variables&quot;];

# add an external domain header to all email not coming from our own domains
if allof(
  not address :domain :is [&quot;from&quot;] [&quot;example.com&quot;],
  not header :contains &quot;Subject&quot; [&quot;[External Email]&quot;]
)
{
  addheader &quot;X-External-Domain&quot; &quot;This Message originated outside of mind.&quot;;
  # Match the entire subject ...
  if header :matches &quot;Subject&quot; &quot;*&quot; {
     # ... to get it in a match group that can then be stored in a variable:
     set &quot;subject&quot; &quot;${1}&quot;;
  }

  # We can't &quot;replace&quot; a header, but we can delete (all instances of) it and
  # re-add (a single instance of) it:
  deleteheader &quot;Subject&quot;;
  # Append/prepend as you see fit
  addheader :last &quot;Subject&quot; &quot;[External Email] ${subject}&quot;;
  # Note that the header is added &quot;:last&quot; (so it won't appear before possible
  # &quot;Received&quot; headers).

}</pre>
Replace <code>example.com</code> with your Zimbra domain name, you can also add additional trusted domains where you do not want the external email warning to be displayed using:

<pre>not address :domain :is [&quot;from&quot;] [&quot;example.com&quot;,&quot;supermodel.com&quot;,&quot;gamer.tech&quot;],</pre>
Enable it on a test account using

<pre> zmprov mc default zimbraSieveEditHeaderEnabled TRUE
cat /tmp/myfilters |xargs -0 zmprov ma test@example.com zimbraAdminSieveScriptBefore</pre>
Example screenshots when receiving email from outside/external domains:

<blockquote>[[File:screenshots/external-email-warning.png|image]] ''Email with validated DKIM and [External Email] added to the subject.''
</blockquote>
<blockquote>[[File:screenshots/gmail.png|image]] ''Also works with GMail.''
</blockquote>
<blockquote>[[File:screenshots/i18n-subject.png|image]] ''Also works in languages with a different alphabet''
</blockquote>
= Gotchas =

# Modifying the message like this will break DKIM in case the user wants to re-validate DKIM manually after it was verified and the subject was changed, but not many people do this.
# This is in itself not a protection against spoofing, you will have to reject email with a FROM domain that comes from untrusted locations, but that should have been done anyway.
# In a multi-tenant environment, it would be best to configure this on the domain. Even if domains are on the same environment, they may be external to one another!

= Further reading =

* https://www.sparkpost.com/momentum/3/3-reference/sieve-syntax-basic/
* https://blog.zimbra.com/2021/07/zimbra-skillz-using-sieve-filters-on-zimbra/


{{Article Footer|ZCS 8.8.15 ZCS 9.0||2021-09-15}}