Failure to comply with the policy may cause loss of data. Failure to comply with quota warnings may cause loss of e-mail. While the system will allow you to temporarily exceed your mail Inbox limit somewhat, you should remove any excess as soon as possible. Delete messages that you don't need, especially large messages.
Messages containing attachments such as pictures can be quite large.
These are good candidates for deletion. Note that the mail program does not delete these messages immediately, but merely marks them for deletion. Save these settings. Make sure this option is unchecked. Check your software documentation for information on how to change this setting.