Adjust email space for email account in Smartermail v16
1) Log in 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
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3) Select the Accounts on the Left panel
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4) Click on the email account from email account list
5) Edit the Mailbox Size Limit (MB) and Save it
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You can read more here.