Here is a video on how to change the mailbox quota of a mail account from within Plesk 10.
By default, the mailbox quota is set to whatever you first entered when setting up the domain. If you are using pre-set templates this can be as little as 10MB. If you are getting notifications of approaching your quota, it does not mean that you are approaching your total hosting space limit, instead, you can simply increase the quota by following the steps in the video below.
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