Added method Folder.isFlagSupported(Flag)

src/com/fsck/k9/controller/MessagingController.java

@@ -2174,14 +2174,7 @@ public class MessagingController implements Runnable {
 
         Store remoteStore = account.getRemoteStore();
         Folder remoteFolder = remoteStore.getFolder(folder);
-        if (!remoteFolder.exists() ||
+        if (!remoteFolder.exists() || !remoteFolder.isFlagSupported(flag)) {
-                /*
-                 * Don't proceed if the remote folder doesn't support flags and
-                 * the flag to be changed isn't the deleted flag. This avoids
-                 * unnecessary connections to POP3 servers.
-                 */
-                // TODO: This should actually call a supportsSettingFlag(flag) method.
-                (!remoteFolder.supportsFetchingFlags() && !Flag.DELETED.equals(flag))) {
             return;
         }
 

src/com/fsck/k9/mail/Folder.java

@@ -153,9 +153,13 @@ public abstract class Folder {
         return null;
     }
 
+    public boolean isFlagSupported(Flag flag) {
+        return true;
+    }
+
     public boolean supportsFetchingFlags() {
         return true;
-    }//isFlagSupported
+    }
 
     @Override
     public String toString() {

src/com/fsck/k9/mail/store/Pop3Store.java

@@ -891,6 +891,11 @@ public class Pop3Store extends Store {
             }
         }
 
+        @Override
+        public boolean isFlagSupported(Flag flag) {
+            return (flag == Flag.DELETED);
+        }
+
         @Override
         public boolean supportsFetchingFlags() {
             return false;