Use domain name for message IDs

app/src/main/java/eu/faircode/email/EntityMessage.java

@@ -187,7 +187,11 @@ public class EntityMessage implements Serializable {
     public Long last_attempt; // send
 
     static String generateMessageId() {
-        return "<" + UUID.randomUUID() + "@localhost" + '>';
+        return generateMessageId("localhost");
+    }
+
+    static String generateMessageId(String domain) {
+        return "<" + UUID.randomUUID() + "@" + domain + '>';
     }
 
     boolean replySelf(List<TupleIdentityEx> identities, long account) {

app/src/main/java/eu/faircode/email/ServiceSend.java

@@ -58,6 +58,7 @@ import javax.mail.MessageRemovedException;
 import javax.mail.MessagingException;
 import javax.mail.SendFailedException;
 import javax.mail.Session;
+import javax.mail.internet.InternetAddress;
 import javax.mail.internet.MimeMessage;
 
 import static android.os.Process.THREAD_PRIORITY_BACKGROUND;
@@ -405,6 +406,14 @@ public class ServiceSend extends ServiceBase {
         if (!message.content)
             throw new IllegalArgumentException("Message body missing");
 
+        // Update message ID
+        if (message.from != null && message.from.length > 0) {
+            String from = ((InternetAddress) message.from[0]).getAddress();
+            int at = (from == null ? -1 : from.indexOf('@'));
+            if (at > 0 && at + 1 < from.length())
+                message.msgid = EntityMessage.generateMessageId(from.substring(at + 1));
+        }
+
         // Create message
         Properties props = MessageHelper.getSessionProperties();
         if (ident.unicode)