Yahoo bans the character string "allah" from all email names.
In other words, your email address can't be firstname.lastname@example.org, but it also can't be email@example.com. However, something tells me that this move is not due to too many people of Irish extraction registering for email accounts.
More is here, with a list of other character strings you can't use in your email address. Ann Coulter will be disappointed to see that "raghead" is one.