you have reached the in-game cap of unique mail recipients

So How did this happen? only been posting to my alts

That means your alt's mailbox is full. All you need to do is make some space in it :')

Well I had this when I tried to send some good good to a friend, then tried to send it from a different character and it works. So idk about the full mailbox thing

It's not about the mailbox being full or not (I have a bank char and I spam it with things all the time and on its mailbox i have so much things that it tells me the mailbox is full and can't show the items past page 4).
I have this problem too now, tried to send something to an alt of mine and it gave me this message even tho I only sent items to my bank char today and no one else.
Maybe it's something that have a connection to how many times did I send ANY mails to anyone?
because then, it would be a bug.

I play WOTLK on Ragnaros and I can confirm this bug or however you want to call it. The error occurs on both of my accounts if i want to send something to a char with full mailbox. When I relatively clear the mailbox the mail can be sent regularly.

In my oppinion it's a bug because an appropriate error message would be "The reciever's mailbox is full". Or even better: handle those mails like for the auction house. Here the piled-up mails arrive after you cleared parts of your mailbox.