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

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.

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

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.