https://tools.ietf.org/html/rfc5321
https://tools.ietf.org/html/rfc5598
https://www.sendinblue.com/blog/what-is-smtp-server/
https://ccm.net/contents/116-how-email-works-mta-mda-mua
https://www.sendinblue.com/blog/what-is-smtp-server/
https://www.makeuseof.com/tag/technology-explained-how-does-an-email-server-work/
https://computer.howstuffworks.com/e-mail-messaging/email3.htm
https://www.mailjet.com/blog/news/dos-and-don-ts-on-setting-up-a-smtp-server/