diff --git a/POSTINSTALL.md b/POSTINSTALL.md
index a1c111c..76c5a0f 100644
--- a/POSTINSTALL.md
+++ b/POSTINSTALL.md
@@ -3,4 +3,8 @@ Accounts are created with the username and the subdomain under which this app is
 to be changed later as it breaks federation. So, you have to install Mastodon in the correct
 subdomain from the get-go.
 
+<sso>
+**note:** Mastodon has [restrictions](https://docs.cloudron.io/apps/mastodon/#username-restriction) on usernames
+that might prevent some users from logging in.
+</sso>