Hello Zimbra Customers, Partners & Friends,
New with 9.0.0 Patch 25 and 8.8.15 Patch 32, you can add an external email warning in Zimbra using the Admin Console. An external email warning message alerts you when you receive email from an external domain. This is done using a Sieve filter, and it can be enabled per account, COS, domain and server.
Here’s a link to the blog describing how to add an external email warning in Zimbra using via the command line using zmprov.
The method described in this blog is different and independent from the external domain warning in the admin guide (which is currently having some bugs).
Sieve via the Admin Console UI
To set up Sieve filter rules for Class of Service (COS) and domains in the Admin Console, click Advanced and scroll down to Sieve filter rules. Here is an example of settings per COS.
For the Sieve syntax, check out this blog post.
- Modifying the message like this will break DKIM if 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 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 per domain. Even if domains are on the same environment, they may be external to one another!