diff --git a/docs/extra-services.md b/docs/extra-services.md
index 545d7bf9e..b39b2380b 100644
--- a/docs/extra-services.md
+++ b/docs/extra-services.md
@@ -23,3 +23,29 @@ The currently supported packages are:
 >
 > We recommend that you use the `composer require` command inside the container
 > in development mode so that recipes can be applied correctly.
+
+## Using Another Email Catcher
+
+The `symfony/mailer` recipe installs Mailpit.
+Any other SMTP catcher works in the same place.
+To use [Mailtrap Local](https://github.com/mailtrap/mailtrap-local) instead,
+replace the `mailer` service in `compose.override.yaml`:
+
+```yaml
+mailer:
+  image: mailtrap/mailtrap-local
+  ports:
+    - "3535"
+    - "3550"
+```
+
+Mailtrap Local needs no environment variables.
+The recipe sets `MAILER_DSN=null://null`, so change that line in `.env`:
+
+```dotenv
+MAILER_DSN=smtp://mailer:3535
+```
+
+Port 3550 serves both the web interface and the JSON API.
+Neither port is published on a fixed host port.
+Run `docker compose port mailer 3550` to get the web interface address.