app/models/user.rb

@@ -131,7 +131,7 @@ class User < ApplicationRecord
 
   def mastodon_address
     return nil unless Setting.mastodon_enabled?
-    "#{self.cn}@#{Setting.mastodon_address_domain}"
+    "#{self.cn.gsub("-", "_")}@#{Setting.mastodon_address_domain}"
   end
 
   def valid_attribute?(attribute_name)

spec/models/user_spec.rb

@@ -41,6 +41,14 @@ RSpec.describe User, type: :model do
           expect(user.mastodon_address).to eq("jimmy@kosmos.social")
         end
       end
+
+      describe "username contains hyphen/dash" do
+        let(:jammy) { build :user, cn: "jammy-jellyfish", ou: "kosmos.org" }
+
+        it "returns the user address" do
+          expect(jammy.mastodon_address).to eq("jammy_jellyfish@kosmos.org")
+        end
+      end
     end
   end