Event ID 12016: There is no valid SMTP Transport Layer Security (TLS) certificate for the FQDN of SERVERNAME.DOMAIN.LOCAL. The existing certificate for that FQDN has expired.
The continued use of that FQDN will cause mail flow problems. A new certificate that contains the FQDN of SERVERNAME.DOMAIN.LOCAL should be installed on this server as soon as possible.
You can create a new certificate by using the New-ExchangeCertificate task.

Cause:
The Self-Signed certificate on MS Exchange Server has been expired.
Solution:
1- Create new Self-Signed certificate by running the command “New-ExchangeCertificate”.
2- Export the new Self-Signed certificate from Personal section in certificate console.
3- Import Self-Signed certificate to Trusted Root Certificate Authorities section.

4- Enable the new generated certificate by executing the command “Enable-ExchangeCertificate -Thumbprint -Services smtp”.
5- Restart ‘Microsoft Exchange Transport’ service.
6- Stop Anti-virus transport agent (additional step).