Email addresses are collected in 'mailing lists'. The list box on the main form shows the currently available mailing lists. 
Mailing lists can be either stored 'Built-in' or as a Linked table or Imported table. Mailing lists can also be based on MS Access queries.
Using Built-in mailing lists allows you to manage email addresses and recipients conveniently.
Using linked table mailing lists can be convenient if someone delivers a new version of the mailing list data, for example a list in Excel. Another advantage is that you have more control on personalization because you can add fields of your own.
Selecting a mailing list is as simple as clicking it in the list box. In the bottom pane tab Mailing List the addresses appear.