Use + in the mail address you use.
For example, lets say my email is abcd@gmail.com and I want 4 diablo accounts (hence 4 b.net accounts) all using this email.
When I create the bnet accounts I use the emails abcd+diablo1@gmail.com, abcd+diablo2@gmail.com, abcd+diablo3@gmail.com and abcd+diablo4@gmail.com. this will register as different email in blizzard, but will cause all emails to be sent to abcd@gmail.com