diff --git a/config/config.exs b/config/config.exs
index d6af65dd5..90731e0c6 100644
--- a/config/config.exs
+++ b/config/config.exs
@@ -129,7 +129,7 @@ config :mobilizon, Mobilizon.Web.Email.Mailer,
     verify: :verify_peer,
     versions: [:"tlsv1.2", :"tlsv1.3"],
     cacerts: :public_key.cacerts_get(),
-    server_name_indication: "localhost",
+    server_name_indication: ~c"localhost",
     depth: 99
   ],
   retries: 1,
diff --git a/config/docker.exs b/config/docker.exs
index 994c20011..7d363172e 100644
--- a/config/docker.exs
+++ b/config/docker.exs
@@ -55,7 +55,7 @@ config :mobilizon, Mobilizon.Web.Email.Mailer,
     verify: :verify_peer,
     versions: [:"tlsv1.2", :"tlsv1.3"],
     cacerts: :public_key.cacerts_get(),
-    server_name_indication: System.get_env("MOBILIZON_SMTP_SERVER", "localhost"),
+    server_name_indication: ~c"#{System.get_env("MOBILIZON_SMTP_SERVER", "localhost")}",
     depth: 99
   ],
   ssl: System.get_env("MOBILIZON_SMTP_SSL", "false"),