- You need to be sending HTML email in order to embed the counter image
- Not everyone reads HTML email -- some people have their mail readers display text only
- Some people read HTML email, but have images disabled by default
Tracking email reads on a commercial email blast is fine and probably even expected. Beyond that, use your judgment when deciding whether or not to track email reads. There's an expectation of privacy that comes with email which doesn't apply to web surfing. People expect to be tracked online, but they don't necessarily expect to be tracked when viewing non-commercial email, but how you set up your tracking is entirely your decision.