421 4.4.2 Message submission rate for this client has exceeded the configured limit.
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.
Get-ReceiveConnector SERVERNAME\* | FT Name,bindings,MessageRateLimit
Also verified in load balancer rate limit is set to unlimited. Lets see how to check the same in F5
Application services _ Applications _ smtp
Connection limit is set to Unlimited.
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.
Get-ReceiveConnector "*\Client Proxy*" | FT Name,*RateLimit*
Get-ReceiveConnector "*\Client Proxy*" | Set-ReceiveConnector -MessageRateLimit 30