梦想照进现实

In my case Application was using Port 25 and using the app custom connector on load balanced Exchange 2016 servers and message rate limit was set to unlimited.

Solution –

Late realized its the Client proxy connector causing the bottleneck. Increased from 5 which is the default value . Changed to 30.

NOTE  : Below Commands sets the rate limit for all Exchange servers in the environment. Use Servername\ for specific server.

if you have issue on Port 587 IMAP mail submission- you should increase Rate Limit on below connector