Often times we have customers who exceed the 15 GB space limit on Google Apps. A lot of times this happens because of them receiving super large file sizes and those files taking up way too much space.
You can search your gmail/google apps account easily for big files and delete them. Click the links below to search your inbox.
Once you search for the emails and see the emails taking up your space, simply select all of the e-mails and delete them to instantly clean up your inbox.