Some ads in your inbox my resemble an actual email. They are nothing to worry about.
Ads in your inbox resembling actual emails are harmless - they are just ads, necessary to offer mail.com services for free. These ads don't affect your quota limits, and advertisers don't have any access whatsoever to your account or your data.
You can remove these ads by clicking their x button.
https://www.mail.com/mail/premium/ customers enjoy an ad free experience.