Several users in our domain keep getting a message that their email account storage limit is about to be reached. 

For instance, my own email account has only 91 emails and storage details show as follow:

Folder 1%

This account 97%

Assigned 5 GB

I could even delete all my emails and in a few days I'll get the warning message.

Is there a way to correctly clear/purge the storage?

Regards