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I don't know why they didn't implement it especially since the function is basically there. I don't believe because of data limits, they could limit the amount of data the message is plus they limit the amount of transactions you can make in a time period so why not do the same for that. It seems it might have been forgotten, I don't know how else to explain it otherwise.

Well, hopefully we'll be able to find out what the issue is or will be able to implement a solution fairly rapidly. I think it would add enormous value to the network/Steemchain.

Perhaps if there are concerns about private spam....private messages have all rewards disabled.

Yes but a way to block users from sending transactions, or otherwise from receiving transactions/messages from certain users in that regard seems like a natural solution to preventing such spam.