Post replies are being worked on to be reintroduced and the fanmail system is being dropped in favor of an chat function. Accounts with the update can no longer send fanmail but your existing messages will still be in your inbox.
There is no word on when post replies will be back, simply that they will and they are going to be improved when they are.
You can disable chat messages from nonmutuals and blogs with multiple admins and private blogs are not capable of sending IMs but sideblogs are.
AKA you can now message someone from a sideblog without giving away your other urls.
You can also delete a conversation or block a person from messaging you from the messenger itself. Unless the person has editted their settings to only receive IMs from mutuals, is a private blog, or is an admin of a group blog, you can message anyone on tumblr.
#bluespace the update looks like this.
That is the situation. Please stop circulating a bunch of incorrect posts because you want to be mad at staff when, really, this is actually a sound and functional update that the userbase has wanted for a very long time.
May's Blog 