Exchange 2016 sudden sending to Gmail & Yahoo errors

Please help. Our Exchange 2016/Server Domain (users email) delivery suddenly was unable {delayed/undeliverable status) to send to Gmail and Yahoo users/accounts only. the sending issue started last Jan.8 for Yahoo accounts and for Gmail started only last Jan 12, 2020, to date.

We have no exchange server configuration activities in the past years and email sent to all external domains was successful since then. MXToolbox Diagnostics also shows no IP Blacklist recorded and Email Server Connectivity Test shows no issue.

Delayed error:

Remote Server at gmail.com (173.194.77.27) returned '400 4.4.7 Message delayed'
2/13/2020 8:40:10 PM - Remote Server at gmail.com (173.194.77.27) returned '451 4.4.0 Error encountered while communicating with primary target IP address: "421 4.2.1 Unable to connect." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts. The last endpoint attempted was 173.194.77.27:587'

Undeliverable error:

Remote Server at gmail.com (173.194.77.26) returned '550 4.4.7 QUEUE.Expired; message expired'
2/13/2020 10:15:46 PM - Remote Server at gmail.com (173.194.77.26) returned '451 4.4.0 Error encountered while communicating with primary target IP address: "421 4.2.1 Unable to connect." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts. The last endpoint attempted was 173.194.77.26:587

Thank you for your assistance.

Regards,

Rodel