When the message window for email composition is open (via composing, replying or forwarding), you can press and hold the side speech button and say a contact name (in the To, Cc, and Bcc message address fields) provided you have agreed to upload your contact names. You can simply press and hold the side speech button to dictate in the subject and Body fields (the cursor must be in that composition field) to dictate any free form text.
Example: With cursor in the "To" Address field say "John Smith", then with cursor in the Subject field say "Today’s meeting" and with the cursor in the Body field say "I am running late for the meeting. I will join in 10 minutes".
Tip: It's better to speak one or two sentences at a time and verify the results rather than dictate a whole paragraph at once.