Free email services are financed by advertising. This means that users are often confronted with advertising in their inbox or user interface. Even if you do not pay for some email services, you pay with your data. This is because your data is sold on. The question here is how much your data is worth to you.
With free services, users often have less control over their data. The data collected is used and sold for advertising purposes, which compromises user privacy.
Security-conscious users should never use a free email service. Take care of your data!