The short answer: email accounts have always been a dumpster fire, and we won’t torture people again by bringing it back. I posted a very expansive reply about why it was removed, and why it won’t be brought back.
Just note that sending emails from websites and email accounts are different systems. Even when we had email accounts, email sending from websites was restricted. The reason for this, like you said, was to prevent criminals from abusing our servers to send spam email.
But then I hope you can also understand why installing a banner is not really enough to stop the spammers. A spammer could just setup a blank page with the footer logo, which would never be seen by anyone. They could then send unrestricted email to anyone to funnel traffic to their scam site on another domain.
Email providers would catch onto this and ban our network (again), meaning nobody can reliably send email.
I can’t ask iFastNet to go through the effort to build a system like that which is so trivial to exploit.
This should be working already. Like the KB article says, PHP mail is extremely limited, but it’s there. Registration emails for popular CMS are generally whitelisted.
Activity notifications are not allowed because they can get very spammy very quickly if you have a large number of registrations, reasonable amount of activity and send unsolicited notifications to every subscriber about every post, like and share.