feat: Add system message when provider does not allow unencrypted messages (#5161) (#5195)

close #5161

![Screenshot from 2024-01-19
19-56-09](https://github.com/deltachat/deltachat-core-rust/assets/39526136/27ecdd9b-1739-410b-bb26-80d5bdbbc39a)

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Co-authored-by: bjoern <r10s@b44t.com>

src/mimeparser.rs

@@ -176,6 +176,10 @@ pub enum SystemMessage {
     /// "%1$s sent a message from another device."
     ChatProtectionDisabled = 12,
 
+    /// Message can't be sent because of `Invalid unencrypted mail to <>`
+    /// which is sent by chatmail servers.
+    InvalidUnencryptedMail = 13,
+
     /// Self-sent-message that contains only json used for multi-device-sync;
     /// if possible, we attach that to other messages as for locations.
     MultiDeviceSync = 20,