I noticed recently in the Event Viewer that there are a number or errors with the source MSExchangeTransport. It is usually just 2 errors but they show up every 5 minutes and have the IDs of 12015 and 12016. Below are descriptions. Mail seems to be going out ok so I'm not sure what the issue is.
An internal transport certificate expired. Thumbprint:2649DA2E6EDFDE9F89021FCAE03EA420FE74A11E
There is no valid SMTP Transport Layer Security (TLS) certificate for the FQDN of 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 domain.local should be installed on this server as soon as possible. You can create a new certificate by using the New-ExchangeCertificate task.