[IMail Forum] Mailbox size managability question

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We have a client who is thinking about offering free email to his users and has asked a question I haven't come across yet.  He's talking about up to 100,000 free accounts, but he wants to be able to offer them a larger mailbox size for a fee.  Has anyone done anything like this with Imail?  And if so how did you implement it?  I think some kind of groupware type option would be the best, a way to add a user to a "group" that has larger limits on their mail boxes.  But I'm open to any suggestions anyone has.   Mike Achenbach