If you refer admin 2FA, yes, but just wanted to be careful.
domain.com/mail just looks a bit nicer without box.xxxxxx.com/mail.
tried a few configurations but failed, i can redirect, but /cloud can only reside on box.domain i think.
the other way i can think of, is you creating index.html in your domain.com www folder,and create an iframe to box.xxx.com/cloud
PS: you know you can create your box as mail.something.com rather than box.something.com right?
Have you checked whether Nginx is having a problem parsing the config?
e.g. run service nginx status on the server and see if it tells you anything about config errors. I found a problem using it recently when I was editing custom.yaml to redirect mail.example.com -> example.com/mail.