What error code indicates that the mailbox is full?

The error code indicating that the mailbox is full is 452. This code specifically means "Requested action not taken: insufficient storage space," which typically happens when a user attempts to send an email to a recipient whose mailbox is full and cannot accept any more messages.

In the context of email servers, this response indicates that the server is unable to process the request to deliver a new message because doing so would exceed the storage limits set for the recipient's mailbox. Understanding this code is crucial for troubleshooting email delivery issues, as it helps identify when the failure is due to limitations on the receiving end rather than issues within the sender's server or network.