Update copy.go

internal/imapx/copy.go

@@ -231,11 +231,18 @@ func streamOne(src, dst *imapclient.Client, dstFolder string, uid imap.UID, flag
 	}
 
 	appendCmd := dst.Append(dstFolder, int64(len(body)), &imap.AppendOptions{Flags: keepFlags(flags), Time: internalDate})
-	if _, err := io.Copy(appendCmd, bytes.NewReader(body)); err != nil {
+	// Append acquires go-imap's per-client encoder mutex and holds it until
-		return err
+	// Close() calls enc.end(). Close() MUST run on every path: if io.Copy
+	// fails mid-write (server stall, idle timeout), returning without Close()
+	// leaks the mutex and the NEXT Append on this client deadlocks forever on
+	// beginCommand. Close() is idempotent and always releases the lock.
+	_, copyErr := io.Copy(appendCmd, bytes.NewReader(body))
+	closeErr := appendCmd.Close()
+	if copyErr != nil {
+		return fmt.Errorf("append body uid %v: %w", uid, copyErr)
 	}
-	if err := appendCmd.Close(); err != nil {
+	if closeErr != nil {
-		return err
+		return closeErr
 	}
 	_, err := appendCmd.Wait()
 	return err