If one day you notice in your contact book a list of email addresses you haven’t added, it is most likely related to Google Contacts settings.
Google automatically saves the email addresses of people you interact with.
When it happens, they all get added to the “Other contacts” folder in Google Contacts.
The good news is that you can disable this function:
Open your Google account settings -> People & sharing -> disable “Contact info saved from interactions.”
To get rid of existing contacts with email addresses, delete them from the “Other contacts” folder. Just press down on one of the contacts and tick all of them on the list.