Zimbra SkillZ: Add an External Email Warning in Zimbra

Hello Zimbra Customers, Partners & Friends,

In today’s blog, I’ll show you how to add an external email warning message in Zimbra when receiving an email from an external domain. This is done using a Sieve filter, and it can be enabled per account, COS, domain and server.

First, create /tmp/myfilters

  su - zimbra
  nano /tmp/myfilters

With the following contents:

require ["fileinto", "reject", "tag", "flag", "editheader", "variables"];

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

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


Replace example.com with your Zimbra domain name. You can also add additional trusted domains where you don’t want the external email warning to be displayed using:

not address :domain :is ["from"] ["example.com","supermodel.com","gamer.tech"],

Enable it on a test account using

 zmprov mc default zimbraSieveEditHeaderEnabled TRUE
cat /tmp/myfilters |xargs -0 zmprov ma test@example.com zimbraAdminSieveScriptBefore

Here are some example screenshots when receiving email from outside/external domains:

Email with validated DKIM and [External Email] added to the subject

Also works with Gmail

Also works in languages with a different alphabet


  1. Modifying the message like this will break DKIM. Users can re-validate DKIM manually after it was verified and the subject was changed, but not many people do this.
  2. This is not a protection against spoofing. You have to reject email with a FROM domain that comes from untrusted locations, but that should’ve been done anyway.
  3. 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.


Stay safe,
Your Zimbra Team

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5 Responses to Zimbra SkillZ: Add an External Email Warning in Zimbra

  1. Murat Apak November 3, 2021 at 1:09 PM #

    Hello Barry, i have a some question. I add this script a user incoming mail added to subject External tag everytinh is fine, but i reply mail and again write a answer this time mail added second External Tag, do yu have a solution this issue

    • Barry de Graaff November 8, 2021 at 10:47 AM #

      Thanks, I have added a check to the Sieve script so that it will not put duplicates of the external email warning!

  2. Murat Apak November 9, 2021 at 10:04 PM #

    Thank you for quick reply. Last question, i need a add few domain how should I use it?

    • Barry de Graaff November 10, 2021 at 6:48 AM #

      not address :domain :is ["from"] ["example.com","supermodel.com","gamer.tech"],

  3. Murat Apak November 10, 2021 at 4:17 PM #

    Thank you Barry.

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