Recipents & clients in exchange server 2013

Mailbox enabled user:  A user account in AD with mailbox & email address in exchange server.

Mail enable user: A user account in AD with external email address with out mailbox in exchange server.

 Note:this user can login into our pc and can communicate with his external email address.only required internet for that system

Mail contact: A contact in AD with external email address.

Note:Assume if we have 500 clients. its difficult to remember. inspite of us the exchange will remember the address.

Distribution Group: A group with email address used for distributing emails to its members .

Dynamic distribution group: It is a mail-enabled groups in which the membership list is calculated based on user attributes when a message is sent to the group

see my video playlist how to create this
https://www.youtube.com/playlist?list=PL7NcipFSl_a_DV0mH6SKvBl2UlSCbWeRJ