Service for sending e-mail during registration?

Question

Email Marketing

You need to send a few registration letters 20-60 per day. There are two features:
1) Letters to new users, they have not yet subscribed to the mailing list and have not expressed a desire to receive letters
2) Someone can make mistakes and letters can be sent to non-existent or other people's addresses, there is an element of sabotage
How mail services and API look at it what third party service is better to use?

Answer 1

https://www.mailgun.com/

Answer 2

How do postal services look at it?

They look normally.
With elementary protection against spam in forms and the correct configuration of the mail server.

Answer 3

Transactional emails can be sent through UniOne . You can read more about it in this article .