WotLK You have reached the in-game cap of unique mail recipients

[Shambler]

What is the cap? For how much time I'm locked?
The funny thing, most of my mails are between characters in the same account, and as I read on Blizzard forums its not supposed to be counted for cap.
The only other mails I can think about are mails to people inside my guild, which also shouldn't be counted for cap.

[Anyone0]

That error message usually appears when the recipient has too many mails in mailbox (around 100).

[Obnoxious]

If you searched for that exact message on retail Forums, you'd find that there's an automatic temporary ban on sending mails when over a certain amount has been sent in a determined period of time.