The maximum amount of space provided to an individual email box is known as email quota. This sets the maximum amount of space usable for that particular email address which includes messages (and attachments) stored on the server in the inbox, sent items, trash, etc. Note that email message file attachments can take up 5-10M per attachment, especially if it's a video or large graphic file. If an email box is over quota, a sender to that address will receive a rejection error message.