Web bug is a common technique designed to monitor who is reading a web page or email message. It is also known as clear GIF, 1-by-1 GIFs or invisible GIFs.
Web bug is a simple thing: a graphic (generally 1 by 1 pixel size) is inserted into an HTML page (IMG tag) linking an external site. This way spammers could find out if a particular email message has been read and hence check the validity of an email account. This technique is also very common in websites to provide hit information about visitors, etc.
How to avoid web bug on you email?
- Disable HTML emails from your email client (accept only plaintext)
- Some email clients disable downloading images by default. Check your email client