Server certificate was rejected by the verifier because of an unknown certificate authority

Please help me regarding this issue. I searching about this but i don’t find proper explanation and guide.

Here is log details:

*** Inner Exception ***
Rebex.Net.TlsException: Server certificate was rejected by the verifier because of an unknown certificate authority.
at Rebex.Net.TlsHandshake.VerifyCertificate(String commonName, CertificateChain certificate)
at Rebex.Net.TlsHandshake.OnHandshakeReceivedClient(Byte[] buffer, Int32 offset, Int32 count, Handshake h)
at Rebex.Net.TlsHandshake.OnHandshakeReceived(Byte[] buffer, Int32 offset, Int32 count)
at Rebex.Net.TlsBase.ProcessHandshake(Byte[] buffer, Int32 offset, Int32 length)
at Rebex.Net.TlsBase.Process()
10:59:14.285 [69] INFO: Worker completed. No exception catched.
10:59:14.313 [5] INFO: MailStore_.FolderGetChildFolders took 00:00:00.0021738
11:00:43.862 [1] INFO: ServiceCommandCallbackEx has been called by the service control manager (command = SERVICE_CONTROL_STOP).
11:00:43.862 [1] INFO: SetServiceStatus(currentState = StopPending, checkPoint = 0, waitHint = 10000, controlsAccepted = 0)
11:00:43.866 [14] INFO: InternalStop has been called (previousStatus = Running, reportPendingStatus = True).
11:00:43.870 [14] INFO: Stopping MailStore Server…
11:00:43.876 [14] INFO: SetServiceStatus(currentState = StopPending, checkPoint = 1, waitHint = 10000, controlsAccepted = 0)
11:00:43.876 [14] INFO: Executing action…
11:00:43.876 [14] INFO: SetServiceStatus(currentState = StopPending, checkPoint = 2, waitHint = 10000, controlsAccepted = 0)
11:00:43.887 [6] INFO: Stopping TCP Server listening on 0.0.0.0 port 8460…
11:00:43.887 [6] INFO: Stopped TCP Server listening on 0.0.0.0 port 8460.
11:00:43.887 [6] INFO: Stopping TCP Server listening on :: port 8460…
11:00:43.887 [6] INFO: Stopped TCP Server listening on :: port 8460.
11:00:43.962 [6] INFO: Disposing MailStoreAccess
11:00:43.964 [6] INFO: Closing archive…
11:00:43.965 [6] INFO: Closing all open file groups…
11:00:43.966 [6] INFO: Closing file group 1…
11:00:43.989 [6] INFO: Lockfile C:\MailArchive\2017-05\Content\MailStoreContent.lock deleted.
11:00:43.990 [6] INFO: Lockfile C:\MailArchive\2017-05\Index\MailStoreIndex.lock deleted.
11:00:43.992 [6] INFO: Writing statistics of file group 1…
11:00:43.998 [6] INFO: Statistics written successfully.

This community is for MailStore Home only.
Therefore I kindly ask all MailStore Server customers to use one of the the support resources listed under https://cs.mailstore.com/