451 5.7.3 STARTTLS is required to send mail

 Cert hết hạn làm mail từ 365 về On-prem lỗi

Reason: [{LED=450 4.4.317 Cannot establish session with remote server [Message=451 5.7.3 STARTTLS is required to send mail] 

Get-ExchangeCertificate | fl Subject,Issuer,Thumbprint

$cert = Get-ExchangeCertificate -Thumbprint THUMBPRINT_CERT

$tlscertificatename = "<I>$($cert.Issuer)<S>$($cert.Subject)"

Set-ReceiveConnector "Default Frontend XXX" -TlsCertificateName $tlscertificatename

Restart-Service MSExchangeFrontEndTransport
Restart-Service MSExchangeTransport