The trouble with that solution is that email is not an instant protocol. It's usually instant, but SMTP RFCs give mail servers up to 72 hours to deliver before

The trouble with that solution is that email is not an instant protocol. It's usually instant, but SMTP RFCs give mail servers up to 72 hours to deliver before they must send back a delay notification. We use Google Apps to host our email, and I've seen plenty of occasions where their systems don't deliver mail immediately. This "issue" used to generate a lot of calls when I was doing freelance consulting. "Bob sent me an email over 20 minutes ago and it's still not here." I'd get those calls all the time. Imagine you're trying to get logged in somewhere and you have to wait an hour or two for an email to show up.