One of the new features in El Capitan and iOS 9 is “Proactive,” which supposedly helps you find email addresses and other info automatically. It doesn’t always work great though. Thankfully, Macworld points to where you can turn it off.
Proactive is supposed to automatically skim emails to pull contact info to make it easier to get in touch with people. This is useful sometimes, but other times it just sits there suggesting email addresses from some random person you don’t care about or worse, pulls up old emails for people for no apparent reason. For example, when I type in my own name, Mail suggests eight different email addresses, including some I haven’t used since college. To clean this up and only allow Mail to suggest emails from your actual Contacts, you just need to toggle a couple settings.
On Mac
Launch Contacts, then head to Contacts > Preferences > General. Uncheck the “Show Contacts Found in Mail” option.
On iOS
Head to Settings > Mail, Contacts, Calendars, and uncheck the box marked “Contacts Found in Mail.”
With those settings disabled, the only autosuggested recipients will be the email addresses you actually have in your address book.
How to disable unconfirmed email suggestions | Macworld
via Lifehacker
Disable OS X and iOS Contact Email Suggestions to Clean Up the Recipient Field