E-Mail for all your mailboxes shares the quota allocated to you for your web hosting account.
For example, the Bronze hosting plan currently comes with 10GB of storage; so if you only use 1GB of storage for your website, then you get 9GB of storage for your e-mail accounts!
If you need even more space than that, just let us know, and we can either upgrade your account to the next level, or add-on additional storage space.