By default, we provide 100MB per email account. Unfortunately it's not possible to increase the size for individual accounts.
We suggest either using a mail client such as "Outlook" or "Mail" to download and store your emails, or, use a web based application such as Gmail. We can change your MX Records to Google's which will allow email under your own domain name to be managed from your current Gmail account directly.
Please see more information on setting up Gmail as your email client.
