How can we verify that the certificate is properly assigned to SMTP

Follow https://learn.microsoft.com/en-us/exchange/architecture/client-access/renew-certificates

• For certificates that were issued by a CA, verify the certificate request requirements of the CA. Exchange generates a PKCS #10 request (.req) file that uses Base64 encoding (default) or Distinguished Encoding Rules (DER), with an RSA public key that's 1024, 2048 (default), or 4096 bits. Note that encoding and public key options are only available in the Exchange Management Shell.
• To renew a certificate that was issued by a CA, you need to renew the certificate with the same CA that issued the certificate. If you're changing CAs, or if there's a problem with the original certificate when you try to renew it, you need to create a new certificate request (also known as a certificate signing request or CSR) for a new certificate. For more information, see Create an Exchange Server certificate request for a certification authority.
• If you renew or replace a certificate that was issued by a CA on a subscribed Edge Transport server, you need to remove the old certificate, and then delete and recreate the Edge Subscription. For more information, see Edge Subscription process.

How do you know that you successfully created a certificate renewal request?

To verify that you have successfully created a certificate renewal request for a certification authority, perform either of the following steps:

• In the EAC at Servers > Certificates, verify the server where you stored the certificate request is selected. The request should be in the list of certificates with the Status value Pending request.
• In the Exchange Management Shell on the server where you stored the certificate request, run the following command:

Get-ExchangeCertificate | where {$_.Status -eq "PendingRequest" -and $_.IsSelfSigned -eq $false} | Format-List FriendlyName,Subject,CertificateDomains,Thumbprint

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hth

Marcin