In a commercial world, everything comes with a price tag. But when your privacy is at stake, you would certainly like to take some action; though it's not your bank details or credit card information Apple is encroaching upon, as a user you won't entertain a slight intrusion in your personal space.
It has been found that iOS 9 scans your email whenever an unknown caller gives you a call. The OS tries to check whether the caller has ever emailed you earlier; it also tries to match up with the number if that person has mentioned in signature.
How to Turn Off Contacts Found in Mail Feature in iOS 9 on iPhone and iPad
Step #1. Launch Settings app on your iPhone → Tap on Mail, Contacts, Calendars.
Step #2. Scroll down and toggle Contacts Found in Mail OFF (This option is below Default Account).
Your email app is one of the most personal things for you, and if there is any invasion from outside, you should stop that immediately. The method mentioned here will certainly help you disable contacts found in mail feature in iOS 9.
From now on wards, your emails will not be scanned when you receive a call from unknown number. This how-to can be helpful to you.
You may also like to read: how to annotate & markup email attachments in iOS 9 on iPhone and iPad, email your WhatsApp chat from iPhone, save email attachment from mail app to iCloud drive in iOS 9, and read emails on Apple Watch.