why not add another function to handle that?

def with_random_bytes(%Email{} = email) do
  Map.put(email, :bytes, random_bytes())
end

Then in your context or anywhere else you need it can always just call it like:

%Email{}
|> Email.with_random_bytes()
|> ...

There’s no difference, really.

def with_random_bytes(struct) do
  Map.put(struct, :bytes, random_bytes()
end

…

%MyStruct{email: email}
|> MyStruct.with_random_bytes()

However, I think new is the way to go. Don’t overcomplicate things. Document that new structs should be created using the initializer in order to set dynamic defaults.

Are Map.new, Range.new, and MapSet.new non-idiomatic?

100% agree, Session.new(email) or Session.new(opts) with documentation feels right IMO.