As standard your MM Platform comes with 1000mb of storage. This space is filled by anything stored on your Platform for example videos, photos and documents as well as emails (if you have set up your email addresses so that they are held in a Mailbox).
If you have created any email addresses within the Platform (so that they are held in a Mailbox) then as standard each email address is given 100mb of space on the server. These can be increased to 1000mb of space per email address free of charge but do note that as advised above, each Platform is allocated a total amount of space of 1000mb and therefore you may want to add on Additional storage if you do want to increase the size of each email mailbox. More information about Additional storage can be found in our What is the additional Storage space extra? help guide.
You can check how much storage you have left on your Platform by going to the Admin dashboard and then selecting the Hosting Account Status page.