diff --git a/README.rdk.md b/README.rdk.md
index d76494a..3e23482 100644
--- a/README.rdk.md
+++ b/README.rdk.md
@@ -17,6 +17,10 @@
 IdentityFile ~/.ssh/<yourkey> #optional, if you want to specify a specific private key
 ```
 
+NB: Due to technical limitations, usernames can not contain the '@' symbol.
+Sky users will replace the '@' in their username with a '\_' for the
+`<youruser>` placeholder above. E.g., user.name@sky.uk becomes user.name_sky.uk.
+
 You can test your access with:
 ```
 ssh-keyscan -t rsa -p 29418 rdkgerrithub.stb.r53.xcal.tv >> ~/.ssh/known_hosts