I am sure a lot of you guys are using your primary gmail to join websites that provide certification. There is a chance that they will use your email for marketing and sorts.
While using primary gmail to sign up. Instead of just entering email, put +sitename with your email.
For eg . If you are signing up at example.com then enter your email as [email protected] and you will receive mail at your original gmail.
This will let you know who sold your email to the third party.
Enjoy learning.