Exchange mailhub delivery problems: This message hasn’t been delivered yet. Delivery will continue to be attempted. The server will keep trying to deliver this message for the next days hours and minutes. You’ll be notified if the message can’t be delivered by that time.

Problem: Several Emails getting stuck in the hub ;

Solution:

• It may the DNS setting on the hub not configured or lost;
• Troubleshoot using query analyzer to get into the bottom of the issue

DNS Setting missing in hub :

Go to Hub transport / Select your hub server/properties & check your external DNS is configured;

Since you are there also have a look into the internal DNS lookups and ensure all is good

Further, troubleshoot using the queue viewer and selecting the hub/s