To be pedantic, the correct URL is technically https://mail.google.com/mail/h/, which then expands in practice to https://mail.google.com/mail/u/0/h/ and then gets the jumble of characters near the end added on which appears to be an everything-compliant cache-busting mechanism. When typing the URL manually the "/" after the "h" is required; "mail.google.com/mail/h" will not work.