Hmm... But why should the 'password' URL be so similar to the mail address. Doesn't this mean that if I know your mail address, I can fairly easily guess the secret URL to let me see that inbox?
(Granted that in the most common use case, if I know the email address, I probably know what's in the inbox, since I am likely the person who sent the message. But still, why not make these 2 different random strings?)
A couple more examples:
1QjYwHNM vs 1QjYwHOc
1Qk07A9x vs 1Qk07A9X