Your hosting account has a master storage space limit which is the amount of space that all web pages, databases, file storage, and email boxes combined use. This storage space is determined by your hosting account package.
Each email account can optionally have its own individual limit (which is part of the total storage space limit), which sets the maximum amount of space usable for that particular email address. This includes messages and attachments stored on the server in the inbox, sent items, trash, etc.
At the time of email account creation, the email storage limit can be set or modified at any time thereafter. Note that email message file attachments can take up 5-10 MB or more 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 similar to this:
mailbox is full: retry timeout exceeded