resource limits were exceeded email message

We received the following email stating

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The following resource limits were exceeded on domain 'yourdomain.com.au',
owned by user account 'johnsmith':

Disk space: 1.38 GB used / 1 GB limit

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This could be due to a large amount of emails stored in your emailbox.
Please go to your emailbox through webmail and remove permanently some of your emails. If you have keep a copy of my emails on server ticked in your computers email program eg, outlook, consider turning this off.

OR

you have a large amount of files on your website.
Please go to your website filemanger in your admin account or via FTP and remove permanently some of your files.

OR

if you backup your website files, delete old backs by your admin account or your shopping cart or CMS.

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If you wish to extend your hard disk limit, please contact us