diff --git a/Readme.md b/Readme.md
index 953406d8936a61afd46211912ada5776381ed2c0..bdaf4cfe714cec7c5e379eedd917158363f6818b 100644
--- a/Readme.md
+++ b/Readme.md
@@ -8,6 +8,29 @@ For now only the web interface and SOAP API have been tested.
 
 This image should be combined with a postfix container to actually send mail, this part and the list_configuration have not been tested / documented yet
 
+# What is this repository
+
+This repository provides two docker images that works together to receive and send email to mailing lists : postfix and sympa.
+
+There is an illustration of how they work together:
+
+```mermaid
+graph LR
+    subgraph Sympa
+      sympa
+      nullmailer
+    end
+    subgraph internet
+        relayin
+        relayout
+    end
+    relayin -->|smtp entrant| postfix
+    postfix -->|ssh| sympa
+    sympa --> nullmailer
+    nullmailer -->|smtp| postfix
+    postfix -->|smtp| relayout
+```
+
 # Installation
 
 1. Copy `.env.sample` to `.env` and adapt