If you are experiencing an issue where the IMAP clients are not able to connect on the Exchange Server 2013/2016 server, and when trying to telnet the 143/993 port the connection drops automatically, then this Tutorial may shed some light on your issue. The error message when trying to test IMAP connectivity is listed below.
Microsoft.Exchange.Monitoring.ProtocolException: Authentication failed. The connection is being closed.
Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.Server response while making connection:[]
Read more at source: http://itprocentral.com/imap-inaccessible-on-exchange-server-2013/
