fix dtt

internal/orchestrator/orchestrator.go

@@ -354,6 +354,15 @@ func (o *Orchestrator) runAccount(ctx context.Context, task store.Task, runID in
 		_ = dst.Close()
 	}()
 
+	// Keep the destination connection warm. It sits idle for the whole
+	// source-side metadata scan (Pass 1 of CopyFolder), which on a large
+	// mailbox runs for minutes; without traffic the server drops it and every
+	// subsequent APPEND fails with "use of closed network connection", copying
+	// nothing. Periodic NOOPs prevent that. Only dst needs it — src is
+	// continuously busy scanning/fetching. Bound to actx so it stops with the
+	// account.
+	go imapx.Keepalive(actx, dst, imapx.KeepaliveInterval)
+
 	// Progress watchdog: track the last time we saw scan/copy activity; if it
 	// goes quiet for stallTimeout, cancel the account so the connections close
 	// and this worker unwinds (it would otherwise block forever on a silent