Can you Use ‘Let’s Encrypt’ on Hybrid Exchange?

KB ID 0001770


A few months ago I wrote an article about getting free exchange certificates with Lets ‘Encrypt’. And that still works peachy, I implemented it on my test Exchange 2019 server (in Hybrid mode) and all appeared well. 

Some time later, (remember this is my test network so it’s not in production.) I noticed that mail was not getting forwarded from O365 to the on premises Exchange server. I’ve also had to fix similar problems like this before here.

These were to sort of errors I was seeing;

Error 451 5.7.5 STARTTLS Hybrid

450 4.4.317 Cannot connect to remote server [Message=451 5.7.3 STARTTLS is required to send mail] [] [LastAttemptedIP=x.x.x.x:25] []

Error 451 4.4.0 TLS Hybrid

450 4.4.317 Cannot connect to remote server [Message=451 4.4.0 TLS negotiation failed with error SocketError] [] [LastAttemptedIP=x.x.x.x:25] []

So the problem is obviously something to do with the certificate on the on premises server, so either it’s misconfigured or Microsoft O365 does not like the ‘Lets Encrypt’ certificate.


Firstly I made sure the send and receive connecters were using the correct certificate.

Lets Encrypt Send Connector Hybrid 

Lets Encrypt Receive Connector Hybrid

Get-ReceiveConnector "PNL-MAIL-2019\Default Frontend PNL-MAIL-2019" | fl TlsCertificateName  
Get-SendConnector "Outbound to Office 365" | fl TlsCertificateName  

Note: Above, change the server names and send connector name to match your own.

At this point I was pretty sure that the problem was with that cert so I bought myself a 12 month SSL cert (for the princely sum of $17) and used that instead.

How To Swap Hybrid Connector Certificates

Get the Thumbprint for the NEW publicly signed certificate (Get-ExchangeCertificate). Then use the following PowerShell.

$cert = Get-ExchangeCertificate -Thumbprint XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
$tlscertificatename = "<i>$($cert.Issuer)<s>$($cert.Subject)"
Set-SendConnector "Outbound to Office 365" -TlsCertificateName $tlscertificatename
Set-ReceiveConnector "PNL-MAIL-2019\Default Frontend PNL-MAIL-2019" -TlsCertificateName $tlscertificatename
Restart-Service MSExchangeTransport

Now recheck the connectors again;

Public Signed Receive Connector

Public Signed Send Connector 

Then attempt to re-validate the connector in Office 365, and it works straight away.

Validated Office 365 to On Premises


Related Articles, References, Credits, or External Links

Office 365: Migrating To Exchange Online

Free Exchange Certificate

No Mail Flow On-Premises To/From Office 365

Author: PeteLong

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