Do you mean outgoing spam emails or incoming spam?
The daily sending limits only affect outgoing email, and they're implemented as a failure to resolve, so they do end up in the queue if they can't be returned to the sender, perhaps because the sender email account is full.
Where would you want email to end up if it can neither be delivered nor returned? The RFCs require that you don't throw it away, and even if you don't mind throwing away spam, you still need to be careful because all overlimit email is not spam.
Jeff