The problem? They, along with a significant percentage of your other users, use text-based email clients.
This is the wrong way of sending text-only email:
<!DOCTYPE html> <html> <head> <title>Thanks for signing up!</title> </head> <body><h1>Thanks for signing up!</h1>Click <a href="http://new-registrants.sneakystartup.net/>here</a> to finish your registration. </body> </html>
Thanks for signing up! Click here to finish your registration.
Thanks for signing up! ====================== Click here[1] to finish your registration. 1: http://new-registrants.sneakystartup.net
Thanks for signing up! ====================== Click here to finish your registration: http://new-registrants.sneakystartup.net