CRM 2011 Email Router - Error #26090 in Event Viewer

Good morning,

Does anyone know a solution to the following error:

#26090 - An error occurred while opening mailbox username@domain.com. Microsoft.Crm.Tools.Email.Providers.EmailException: Error executing POP3 command "<Command removed for security> reasons>". The server response was: "-ERR [IN-USE] Temporary authentication failure [bart0050 :2014-06-06 19:51:47]".
   at Microsoft.Crm.Tools.Email.Providers.Pop3Client.ValidateLine(String receivedLine, String command, Boolean hideCommand)
   at Microsoft.Crm.Tools.Email.Providers.Pop3Client.ReadDataWithEncoding(StreamReader streamReader, String command, Boolean singleCommand, Boolean skipValidation, Boolean hideCommand)
   at Microsoft.Crm.Tools.Email.Providers.Pop3Client.ReceiveData(String command, Boolean singleCommand, Boolean skipValidation, Boolean hideCommand, String& encodingCharset)
   at Microsoft.Crm.Tools.Email.Providers.Pop3Client.SendCommand(String command, Boolean singleCommand, Boolean skipValidation, Boolean hideCommand)
   at Microsoft.Crm.Tools.Email.Providers.Pop3Client.SendCommandSecure(String command, Boolean singleCommand, Boolean skipValidation)
   at Microsoft.Crm.Tools.Email.Providers.Pop3Client.AuthenticateClearText(String userName, String password)
   at Microsoft.Crm.Tools.Email.Providers.Pop3PollingMailboxProvider.OpenMailbox()
   at Microsoft.Crm.Tools.Email.Providers.CrmPollingMailboxProvider.Run()