Hybrid Email delivery issues. Last Error: 451 4.7.500 Server busy. Please try again later.

Dear All

i would like to share my experience on the issue below 

Hybrid exchange server 2016 has a huge queue pileup due to my bulk email testing  using Traffic generator . Event though i stopped the test email flow generator , mail queue does n't settle up towards exchange online 

Have resolved the issue, with help of microsoft.

Last error : Last Error: 451 4.7.500 Server busy. 

Identity: CMEX01\32\141026067731

Subject: CRITICAL : Huge connections from common account

Internet Message ID: <ww2e7.wewwe32DAi60r+6awE9R%No_Reply@cloudmonkeys.xyz>

From Address: <>

Status: Ready

Size (KB): 5

Message Source Name: SMTP:Default CMEX01

Source IP: 123.44.9.10

SCL: 0

Date Received: 12/12/2022 1:41:27 PM

Expiration Time: 12/12/2022 1:41:27 PM

Queue ID: CMEX01\3

Recipients:  yt100314@cloudfield.onmicrosoft.com;2;2;[{LED=421 4.4.2 Connection dropped due to SocketError};{MSG=};{FQDN=};{IP=};{LRT=}];0;CN=from ExOnprem to Off365 

Last Error: 451 4.7.500 Server busy. Please try again later from [123.44.9.10]. AS(046262) [IN7EUR03FT052.eop-INR03.prod.protection.outlook.com 2022-12-03T12:50:12.449Z 08DB055743869EFA]

if you guys faced any issue on the queue pileup towards exchange online and the last error whiich you found some thing like Last error : Last Error: 451 4.7.500 Server busy  . Please open a support case with Microsoft. They do the exclusion at the back-end.

https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-online/fix-error-code-451-4-7-500-699-asxxx-in-exchange-online

https://www.alitajran.com/error-code-451-4-7-500-server-busy-email-delivery-issues/