550 4.4.7 Queue expired, Message expired

Hi Guys,

We got NDR for some domains with error code #550 4.4.7 QUEUE.Expired; message expired ##;

So i go to the queue viewer and see some queue stuck there with last error

1. 451 4.4.0 Primary target IP address responded with : "421 4.2.1 unable to connect";

2. 451. 4.4.0 DNS query failded;

Any suggestions for troubleshooting?
If the mails and queues keep failing, will them be deleted or moved to somewhere finally?
As i monitor the queue recently and the queue is always changing, not one specific queue stuck there all the time.

Thanks!

Weicong888