When you start typing an email address in the "To", "CC", or "BCC" section in a new email, you will see email address suggestions in a lookup list. These suggestions are added from your contact list and also the "Known Senders". Known Senders are email addresses that you have sent emails to previously. The icon to the left of the suggested address indicates where the address came from.

You can remove addresses from the "Known Senders" list by swiping left on an address suggestion that has the known sender icon. You can also view/edit the whole known sender list. Open the app settings (View) - Email Management - Known Senders.