For example,the xeams is running at the could server,backend service offline due to outage for several hours or a day. In that case,there maybe too many emails stay at the queue,after the backend server back online,if send one by one,it is really too slow and takes too long time.

If backend server is exchange,it is easily to have below issue:

451 4.7.0 Temporary server error. Please try again later. PRX4
451 4.7.0 Temporary server error. Please try again later. PRX5

If you do not check the log,maybe not know the exchange issue,as telnet exchange 25 port is normal status.This case also cause too many queue.

So extend the retry queue threads is urgently needed function.