1. Login to the web interface as administrator through URL : mail.yourdomainname.com
with administrator username with password
Note:
The email account need to have administrator privilege in order to adjust email
account disk space from Smartermail
2. Go to the last button of the top panel > Domain Settings
3. Select the Accounts on Left panel
4. Click on the email account from email account list.
5. Edit the Mailbox Size Limit (MB) and Save it.