diff --git a/README.md b/README.md
index 76f5d46..d4b2e0a 100644
--- a/README.md
+++ b/README.md
@@ -27,9 +27,6 @@ Via Docker Compose:
     restart: always
 ```
 
-Optionally, set `MAILNAME` environment variable. (By default, Exim will use the
-hostname of the container.)
-
 ### SMTP smart host
 
 In this example, linked containers can connect to hostname `mail` and port `25`
@@ -57,5 +54,23 @@ Via Docker Compose:
       RELAY_PASSWORD: secretpassword
 ```
 
-Optionally, set `MAILNAME` environment variable. (By default, Exim will use the
-hostname of the container.)
+### Environment variables
+
+All environment variables are optional. But if you specify a `RELAY_HOST`, then
+you'll want to also specify the port, username and password otherwise it's
+unlikely to work!
+
+* **`MAILNAME`**: Sets Exim's `primary_hostname`, which defaults to the
+  hostname of the server.
+* **`RELAY_HOST`**: The remote SMTP server address to use.
+* **`RELAY_PORT`**: The remote SMTP server port to use.
+* **`RELAY_USERNAME`**: The username to authenticate with the remote SMTP
+  server.
+* **`RELAY_PASSWORD`**: The password to authenticate with the remote SMTP
+  server.
+* **`RELAY_NETS`**: Declare which networks can use the smart host, separated by
+  semi-colons. By default, this is set to
+  `10.0.0.0/8;172.16.0.0/12;192.168.0.0/16`, which provides a thin layer of
+  protection in case port 25 is accidentally exposed to the public internet
+  (which would make your SMTP container an open relay).
+