To free up your email server space, you need to educate all your users to NOT leave a copy of read mail on the server. This way, once you login into the e-mail server, you will download the emails and clear up the disk space. Normally, the leave mail on server feature is disabled in most email softwares.

In Outlook Express, under Account -> Properties -> Advanced, there is a check box for “Leave a copy of messages on server”. Please uncheck it to stop leaving mail on the server. The other option is to delete the mail through webmail. Mails deleted in webmail are removed PERMANENTLY.