This is probably happening because it’s coming upto your Exchange Server’s 1 year anniversary. When Exchange 2007 first loads it installs it’s own self-signed certificate but I believe this only lasts one year, the idea being that most people replace it with one from a Certification Authority which lasts for longer. Anyhow, you can simply create a new certificate!
First open up the exchange shell and type the command Get-ExchangeCertificate | List
This will show the current certificate and you can see the expiry date of it
Create a new certificate with the command
It will prompt to overwrite SMTP, say Yes
Now if you run
Get-ExchangeCertificate | List
You will see both certificates listed. They each have a thumbprint which is their unique identifier.
You will also see that the old certificate is still being used for IIS.
Enable the new certificate for IIS with the command
Enable-ExchangeCertificate -Thumbprint (new certificate’s thumprint) -Service IIS
Now remove the old certificate
Remove-ExchangeCertificate -Thumbprint (old certificate’s thumbprint)
And you’re done