Send emails from your server, or use a mailing service?

Question

Email

It is necessary for your customers to send email notifications several times a day (like on a toaster a notification about the answer to a question, etc.).
Letters can be up to 10-20 thousand per day. Letters in HTML format. Customers are not required to respond to emails. What to choose for such a purpose?

Answer 1

It is better to do it from your service and do it in such a way that users themselves can both subscribe and unsubscribe from the mailing list.
To do this, you always need to send additional. field: List-Unsubscribe
https://support.google.com/mail/answer/81126?hl=ru
https://geektimes.ru/post/101440/

Answer 2

only mailings, IP can get into the ban server
look for transactional email