diff --git a/infra/email-server/readme.md b/infra/email-server/readme.md
index 4ee2671..3b3c4b9 100644
--- a/infra/email-server/readme.md
+++ b/infra/email-server/readme.md
@@ -1,5 +1,7 @@
 # Email server
 
-
+# Useful docs
+
+https://www.mailgun.com/blog/email/which-smtp-port-understanding-ports-25-465-587/#subchapter-3