[IceDragon200]

Well, I had a comment, but was apparently too long, so I've placed it into a gist for now https://gist.github.com/IceDragon200/b71cafd052ee03f65d1cadc...

For the email validation I would have used an ecto schema, since most cases you won't just be validating an email address in isolation:

  defmodule EmailSchema do 
    use Ecto.Schema

    import Ecto.Changeset

    @primary_key false

    embedded_schema do 
      # here is your type validation right off the bat
      field :email, :string
    end

    def validate(email) do
      %__MODULE__{}
      |> cast(params, [
        :email,
      ])
      |> validate_required([
        :email,
      ])
      |> validate_change(:email, fn :email, value ->
        cond do
          not is_email_address?(value) ->
            [email: {"invalid email address", [validation: :email]}]

          not EmailAddresses.is_available?(value) ->
            [email: {"is unavailable", [validation: :email]}]

          true ->   
            []
        end
      end)
      |> apply_action(:insert)
    end
  end

  case EmailSchema.validate(email) do 
    {:ok, %{email: email}} ->

    {:error, %Ecto.Changeset{} = changeset} ->
      changeset.errors[:email] 
      # Can be all of these in the same list, or be any one depending on the validations
      #=> [{"is required", [validation: :required]}]
      #=> [{"invalid email address", [validation: :email]}]
      #=> [{"is unavailable", [validation: :email]}]
  end