All email accounts have a quota or space limit, which can be a specific quantity of Megabytes ( MB ) or Unlimited.
Once a email account's space usage reaches the space limit it won't receive new emails, but you won't lose any message, they will simply wait for space availability on the email queue.
In order to modify an email account's quota follow these steps:
1 - Access your Hosting Panel
Follow instruccions of How to access your Hosting Panel to access your Hosting Panel.
2 - Click on Email accounts
Look for and click on Email accounts icon.
3 - Click on Modify Quota
Locate your email account in the listing and click on Modify Quota link.
4 - Define the new space limit
Enter 0 for unlimited or a specific quantity of Megabytes ( MB ). Click on Send button.