Can't create a single letter email user

Can't create a single letter email user

My primary email address for my zoho free email account is (some letters replaced with "*"):

dk*@da***********.com

I am trying to create an additional user with email address (also some letters replaced):

d@da***********.com

and I get an error "User already exist in your org"

however, if I try to create:

k@da***********.com

it works.

Why can't I create a user with single letter "d"???

thx for any advice!