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

[krheigh]

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

[Zielona]

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

[Seen14]

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

[Klala]

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.

[irgendjemand]

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.