We shouldn't use deliver or deliver_now.
Always use deliver_later. deliver_now is synchronous and therefore users will have to wait for the email to be sent before receiving an answer. The server may take longer to answer and you really don’t want your users to wait for it.
Also because of the fact that the mail server answers, doesn’t mean that the email has been delivered, and especially that it has been delivered correctly.

So always use deliver_later