As the message indicates, the certificate will expire soon and you have to do something about that. The system will keep bugging about this until you fix it. You can remove the expired certificate a command called remove-exchangecertificate.
I started seeing this event close to 1 year after installing our Exchange 2007 server. The default self-signed cert has to be renewed. A 12017 warning that contains the number of hours you have left to do this precedes this event. I ran the following Exchange PowerShell commands to resolve this issue:
1. get-exchangecertificate | fl
2. new-exchangecertificate -confirm -DomainName servername servername.domain.name -Keysize 2048 -Services SMTP
3. enable-exchangecertificate -Thumbprint LONGHEXNUMBER -Services:None
The first command gives you the list of your certificates. Find the matching thumbprint (long hex number) to see the one that is expiring. Use the parameters shown in that certificate to fill in the fields for your second command. Then, once successful use the third command to disable the expiring certificate from all services.
More info on the parameters can be found at “Creating a Certificate or Certificate Request for TLS “.