If the Recipients Panel's bar is not visible at the right-side of a new email being created, turn it on from the View → Show Recipients Pane menu option.
To add a contact's or company's email address to any of the boxes within the panel, type their name or part of the name into the standard Contact Selector at the top. After the name completes, press the To, Cc, or Bcc buttons to move the person's address down into the address boxes below. Multiple recipients can be added at once, separate multiple entries with a semi-colon.
Email addresses can also be drag-and-drop'ed between panels. For example, if someone is entered into the Bcc field and you would like them instead to show in the Cc field, select the address and drag it up from the Bcc box to the Cc box.
Addresses entered in the Recipients Panel will be shown at the left in the regular To, Cc, and Bcc fields. Once there are too many addresses to be shown easily in those fields, the text of the field will be changed to "multiple recipients" and the individual addresses will only be shown at the right. Since most bulk mailings from the Contacts screen will include a lot of recipients, most times the "multiple recipients" text will be shown right away when doing a bulk mailing.