Why does it happen?
When creating an email account, it gets a predifined storage space. With time this space will overflow.
As the message states, this error happens when the email is about to reach it´s maximun storage space available in the server. Thus is extremely important that we learn how to correctly manage our files stored in the server in order to avoid getting this message. This is a more common issue with IMAP tipe accounts however it can still be found with POP tipe accounts.
How to fix it?
To fix this, we can go to the set up options of our email account.
If you need more information on how to set up your email account in Outlook you can access to this link: https://ecolohosting.com/clients/knowledgebase/206/How-to-Setup-a-POP-E-mail-Address-in-Outlook-2013.html 
In this window, we can go to the delivery options to set a maximum time for our information to be stored in the server, after this time all information will be deleted automatically
Note: Be extremely carefull as this can lead to the loss of important information if you don´t manage local backups correctly.
Unfortunately due to the functionality of IMAP accounts it is not possible to set this configuration, in this case you will need to manually delete mails from your inbox and then delete them again from the "deleted items" tab.
If you need help you can contact us via a support ticket.
