Resolved: [{LED=451 4.4.395 Target host responded with error. -> 421 4.4.1 Connection timed

 Mails stuck in queue, with the following error in queue.

[{LED=451 4.4.395 Target host responded with error. -> 421 4.4.1 Connection timed

the solution was:

Enable-ExchangeCertificate -ThumbPrint "thumbprint of the required new certificate" -services SMTP

Detail:

After updating SSL certificate in Exchange Server 2016 environment, including mailbox servers and edge servers, the mails were getting stuck in queue with the error "[{LED=451 4.4.395 Target host responded with error. -> 421 4.4.1 Connection timed"

after looking into it further, it was found that the new certificate was not enabled for the SMTP service.

So by enabling the new Certificate for the SMTP service, the issue was resolved.

Enable-ExchangeCertificate -ThumbPrint "thumbprint of the required new certificate" -services SMTP