You can change the email mailbox quota through your cPanel control panel which is where you manage your hosting account. If an email address has suddenly stopped receiving new emails, the quota may have been exceeded. However, this guide will explain how this can be increased.
The first step is to log into your cPanel account. If you are unsure on how to do this, please view the following help article - 'How to log into cPanel'.
Once you have successfully logged in, either type email into the search box or scroll down to the 'Email' section as per the below. Select the first option shown 'Email Accounts'.
In the email accounts area, scroll down to find the email account of which you wish to adjust the quota for. Once located, choose the 'Change Quota' option as shown.
This action will then expand a new section below where the email account is shown. You can then adjust the size of the mailbox quota. If you wish, you can set a new value or you can choose 'Unlimited' which will mean the quota should never run out.
Once the adjustment has been made, choose the 'Change Quota' button to complete the process.
You will now see visual confirmation that the change has been completed and confirm what new value has been set.
Changing an email mailbox quota is now complete.