If you set the mailbox size to dynamic, the customer can choose how much of their allocated space they give over to email. The emails are always stored on the email server, regardless of the way you configure the package.
There isn't currently a way to set an overall allowance for email which is separate to the web space. You can choose to allow a fixed number of mailboxes with a fixed size, or dynamic sizing subject to the package disk quota.
The great thing about Enhance is that it's really easy to move websites between servers therefore, if your servers start getting full, you can move customers around to re-balance resources.