diff --git a/README.md b/README.md
index 227da21..ed08bdd 100644
--- a/README.md
+++ b/README.md
@@ -24,4 +24,12 @@ bash /tmp/installer.sh -l 2>&1 | tee ~/output.log
 #### Sources:  
 ```  
 https://linuxize.com/post/set-up-an-email-server-with-postfixadmin
+https://www.howtoforge.com/amavisd_postfix_debian_ubuntu
+https://www.linuxbabe.com/mail-server/setting-up-dkim-and-spf
+https://linuxconfig.org/how-to-change-welcome-message-motd-on-ubuntu-18-04-server
+https://phoenixnap.com/kb/automatic-security-updates-ubuntu
+https://www.digitalocean.com/community/tutorials/ufw-essentials-common-firewall-rules-and-commands
+
+https://www.mail-tester.com
+https://www.emailsecuritycheck.net
 ```  
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