1. The logs usually tell you what certificates are about to expire or has expired
2. Run the commandlet New-ExchangeCertificate to renew Exchange Transport certificates
3. Open MMC and add certificates snap-in, select Computer Account. And then under Personal Certificates, look for the cert and renew using the same key.
4. Once renewed, make sure it is trusted, if not, copy and paste the cert into Trusted Root Certification Authority Certificates