An example of building a user model which uses email as username rather than username

email_as_username_admin.py

from django.contrib import admin
from django.contrib.auth.admin import UserAdmin
from django.contrib.auth.forms import UserCreationForm

from .models import *


class UserAdmin(UserAdmin):
    add_form = BlogUserCreationForm

    add_fieldsets = (
        (None, {
            'classes': ('wide',),
            'fields': ('email', 'first_name', 'last_name', 'username', 'password1', 'password2',),
        }),
    )

    inlines = (BlogUserProfile, )


admin.site.register(BlogUser, UserAdmin)

email_as_username_model.py

from django.contrib.auth.models import *
from django.core import validators
from django.utils.translation import ugettext_lazy as _
from django.db import models


class BlogUser(AbstractBaseUser, PermissionsMixin):
    email = models.EmailField(
        _('Email Address'), unique=True,
        error_messages={
            'unique': _("A user with that email already exists."),
        }
    )
    username = models.CharField(
        _('Username'), max_length=30, unique=True, blank=True, null=True,
        help_text=_('30 characters or fewer. Letters, digits and _ only.'),
        validators=[
            validators.RegexValidator(
                r'^\w+$',
                _('Enter a valid username. This value may contain only '
                  'letters, numbers and _ character.'),
                'invalid'
            ),
        ],
        error_messages={
            'unique': _("The username is already taken."),
        }
    )
    first_name = models.CharField(_('first name'), max_length=30, blank=True)
    last_name = models.CharField(_('last name'), max_length=30, blank=True)
    is_staff = models.BooleanField(
        _('Staff Status'), default=False,
        help_text=_('Designates whether the user can log into this admin '
                    'site.')
    )
    is_active = models.BooleanField(
        _('Active'), default=True,
        help_text=_('Designates whether this user should be treated as '
                    'active. Unselect this instead of deleting accounts.')
    )
    ###
    # the next two fields are custom for my app for email validation
    ###
    is_verified = models.BooleanField(
        _('Verified'), default=False,
        help_text=_('Designates whether this user has verified an email account')
    )
    verified = models.DateTimeField(_('Verified Date'), blank=True, null=True, help_text=_(
        'Stored value of when email activation was completed'
    ))
    ####
    date_joined = models.DateTimeField(_('Date Joined'), default=timezone.now)

    objects = UserManager()

    USERNAME_FIELD = 'email'

    REQUIRED_FIELDS = ['username']

    class Meta(object):
        verbose_name = _('User')
        verbose_name_plural = _('Users')
        abstract = False

    def get_full_name(self):
        """
        Returns email instead of the fullname for the user.
        """
        return "{0} {1}".format(self.first_name, self.last_name)

    def get_short_name(self):
        """
        Returns the short name for the user.
        This function works the same as `get_full_name` method.
        It's just included for django built-in user comparability.
        """
        return self.get_full_name()

    def __str__(self):
        return self.email

    def email_user(self, subject, message, from_email=None, **kwargs):
        """
        Sends an email to this User.
        """
        send_mail(subject, message, from_email, [self.email], **kwargs)

email_as_username_settings.py

# Make sure you declare your custom model in settings

AUTH_USER_MODEL = 'users.BlogUser'