very simple question about username/domain

[Gustavo Vieira Goncalves Coelho Rios]

i am writing a small utilities to play with username/domain under qmail.

I would like to known what are valid email address?
Can some one point me to the URL(s) that specify such a thing?

For instance?
is $%#@nowhere.?4.br a valid email address ?
(i known it's not, i need a formal speficication of what is a valid
email and what is not).


Thanks a lot for your time and cooperation.