Any provider can reject to give the auth code, especially if asked from the admins email address.
If it happens, first of all send a fax or an official communication to the provider asking the auth code.
If they still don't give the auth code to you, you can ask it directly to the Reg. In most cases, the provider and the Reg, aren't the same company.
To know who is the Reg, you only have to put your domain name in one of the whois services on the net, for example: who.is.