For example,xeams is running as front,if the downstream server has some problems for a long time,there are hundreds of emails stay at the retry queue,once the downstream server back online,if send one by one,it is too slow and takes too long time.

Best suggestion:

There is changeable options at the control pannel,by default,the retry thread is set to 1 or 2,for urgent case,administrator can change to be 5-10 and restart xeams to make it works.