Zimbra Tips : How to Add External Email Warning Message

I received a question from my client about how to add an external email warning message in Zimbra when receiving an email from outside. He wants to configure like in Office 365. So, this is how I configure external email warning message in Zimbra.

I am using Zimbra 8.8.15 on CentOS 7 when writing this article.

Create disclaimer-external.conf file

su - zimbra
vi /opt/zimbra/conf/disclaimer-external.conf

Fill it with the following lines

use strict;

$altermime='/opt/zimbra/common/bin/altermime';
@altermime_args_disclaimer = qw(--verbose --pretext=/opt/zimbra/data/altermime/_OPTION_.txt --pretext-html=/opt/zimbra/data/altermime/_OPTION_.html --force-for-bad-html);
$defang_maps_by_ccat{+CC_CATCHALL} = ['disclaimer'];
$allow_disclaimers = 1;
@local_domains_maps = (["."]);
@disclaimer_options_bysender_maps = ({
        '.' => 'external_domains',

},);

1;

Create external_domains.html and external_domains.txt files

vi /opt/zimbra/data/altermime/external_domains.txt

Fill with the following lines

CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.

vi /opt/zimbra/data/altermime/external_domains.html

Fill with the following lines

[code lang=’plain’]

[/code]

Open file amavisd.conf.in

vi /opt/zimbra/conf/amavisd.conf.in

Add the following line above 1; at the end (before the last line)

include_config_files('/opt/zimbra/conf/disclaimer-external.conf');

Save and restart Amavis

zmamavisdctl restart

Below is an example when receiving email from outside/external

NOTE:

1. If you enable the disclaimer in Zimbra by following the Zimbra Wiki: Domain_level_disclaimer_support, your disclaimer text will be put before the message (–pretext). When the default is after the message
2. Please make sure the amavis service is running

Update 2 Dec 2020

If you want to skip adding an external warning for specific domains, create empty files and add to the configuration

Create empty text and html files

touch /opt/zimbra/data/altermime/local_domains.txt
touch /opt/zimbra/data/altermime/local_domains.html

Then add local domains in disclaimer_options_bysender_maps and restart Amavis

@disclaimer_options_bysender_maps = ({
        'example.com' => 'local_domains',
        '.' => 'external_domains',

Update 30 Jun 2021

if DKIM not signed for outgoing message, please try the guide below

Open amavisd.conf.in

vi /opt/zimbra/conf/amavisd.conf.in

put comment (#) on this line “%%uncomment SERVICE:opendkim%% forward_method => ‘smtp:[%%zimbraLocalBindAddress%%]:10030′”. So that like below

## %%uncomment SERVICE:opendkim%%  forward_method => 'smtp:[%%zimbraLocalBindAddress%%]:10030',

Save and restart Amavis

zmamavisdctl restart

Ref: How to Add External Email Warning Message – Prevent Email Spoofing in Office 365

Good luck 🙂