I upgraded from 16.3.6705 to 16.3.6816 and outbound connections to 188.125.73.28:465 fail with Exception: The remote certificate is invalid according to the validation procedure
[2018.09.01]    at System.Net.Security.SslState.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, Exception exception)
[2018.09.01]    at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
[2018.09.01]    at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
[2018.09.01]    at RelayServer.Clients.SMTP.ClientConnectionSync.InitiateSsl(Boolean validateAllCerts)
[2018.09.01]    at RelayServer.Clients.SMTP.SmtpClientSession.Initialize(String #qCb, List`1 #RVi, String #64i)
I've verified the certificate and it's chain within the OS and there are no issues. The certificate has been in place for most of the year and I've not had a problem like this. I've reverted to 16.3.6705 and mail is being sent correctly again.