He says that his inbox space left is 76.4%, but I see that he has -742 characters left in his inbox and red text underneath the sent button that says his inbox is full.
Halp pleeze.
Edit: I don't need to send him a message, it's just looks like a bug that I thought you should know about.
whenever you send a message, when you reply, the message takes into account the previous message as well as your current one
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it thinks you're attaching the previous message in a reply every time, even if you arent, hence the negative inbox space when you haven't even entered a message.
That's the problem, i've had it a few times over the months.
If you go back to the person's profile, and send a new PM (not a reply) then you should be able to send the message.
Also, I think Saved Messages take up some regular inbox space as well.