diff --git a/app/src/main/java/eu/faircode/email/Core.java b/app/src/main/java/eu/faircode/email/Core.java
index 19d26ac858..447ec2fc81 100644
--- a/app/src/main/java/eu/faircode/email/Core.java
+++ b/app/src/main/java/eu/faircode/email/Core.java
@@ -1145,6 +1145,9 @@ class Core {
         if (target != folder.id)
             throw new IllegalArgumentException("Invalid folder");
 
+        if (account != null && account.isWebDe() && "Postausgang".equalsIgnoreCase(folder.name))
+            throw new IllegalArgumentException("'Postausgang' is read-only, use 'Gesendet' instead");
+
         // External draft might have a uid only
         if (TextUtils.isEmpty(message.msgid)) {
             message.msgid = EntityMessage.generateMessageId();