URLs: https://blog.apnic.net/2017/12/12/internet-protocols-changing/ https://hardenize.com

1. SMTP / SMTPs
2. IMAP / IMAPs
3. POP3 / POP3s
4. DKIM
5. RBL
6. UA → MSA → MTA → MDA → mailbox
7. SPAM
8. OpenRelay
9. Kody SMTP 2xy 4xy, 5xy
10. HELO
11. revDNS
12. GreyList
13. SPF (DNS, SPF, TXT)
14. DKIM (było: DomainKeys)
15. DMARC
16. ARC
17. MAIL SRS
18. DANE
19. STS
20. TLS-RPT
21. Blacklists
22. Whitelists
23. Postfix
24. qmail
25. Exim
26. Exchange
27. Outlook / Gmail / Thunderbird
28. SpamAssasin
29. Rpamd
30. Dovecot

Gmail fail: “ProtonMail said that emails sent to Gmail users were “permanently bouncing,” returning error messages to the original sender.” https://twitter.com/ProtonMail/status/1338969496283963392

https://bugzilla.mozilla.org/show_bug.cgi?id=22687 “Opened *21 years ago* Closed 4 months ago”