At a high level, email is two-way (email hosts have to accept your email servers' communication if you choose to send to them), but navigating to a webpage is o

At a high level, email is two-way (email hosts have to accept your email servers' communication if you choose to send to them), but navigating to a webpage is one way (client initiates an HTTP download from the server). Therefore email needs some kind of 'web of trust', since email servers have to trust that other valid email servers regulate their spam and aren't malicious. Not saying the current situation is ideal necessarily.