If your organization’s clients work with POP3 or IMAP4 services dealing with Microsoft Exchange 2003\2007\2010, its recommended to secure the communication and authentication methods using SSL as follows:

1.Connect to IIS manager and on the ‘secure communication tab’ mark the > ‘require secure channel (SSL)’ and the ‘require 128 bit encryption’ checkboxes.
2.On the exchange manager < go to ‘server configuration’ < go to ‘client access'< go to ‘ IMAP properties’ <select the POP3 or IMAP4 tab(one by one) <go to   ‘authentication’ tab < mark the ‘secure logon. A TLS connection is required for the client to authenticate to the server’ checkbox.