Introduction
It is important to understand the distinction between Blackboard's Email and Messages tool. Instructors can make both tools available to students.
Table of contents
Messages Tool
Messages are private and secure text-based communication that occurs within a Course and among Course members. Although similar to email, users must be logged into the Course to read and send Messages. This is for internal communication only within Blackboard. Messages are stored within Blackboard. 
Email Tool
Instructors can send email to all or selected individual Users, Students, Groups, Teaching Assistants, Instructors or Observers. From a Blackboard Learn course, email cannot be sent to anyone who is not a member of the course. Blackboard does not keep a copy of each individual sent item, but if you cc yourself you will be able to see the message content and who it was sent to. Blackboard does not store email.
If you use your mail client outside Blackboard to email students, they will see it in their email accounts, not in Blackboard. Blackboard is not a mail client. 