What are your clients wanting in terms of the functionality that Exchange offers? E.g. if they're just wanted mail synced across all devices, IMAP would be fine. If they're wanting calendars and contacts synced also, then are you wanting to provide the service yourself or refer them to another service (e.g. Google Apps?)
Just trying to get some background context.
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You can resell these kind of services with Atmail Cloud. I am using them for a month now. You have to buy at least 5 mailboxes for $2 and it includes ActiveSync. Their Email online interface looks great.
But I will discontinue being a customer because of their horrible support and that they can't tell me when ActiveSync for Android will work again and many other things.
To the OP, there are many "Exchange like" products out there which to some degree of other will deliver a solution.
Best to find out as much as you can about your client's expectations first though as if they're expecting the same behaviour as a genuine Exchange platform, then a proper hosted Exchange solution is hard to beat.
IMHO from previous experience using these, we keep coming back to Exchange (in our case, delivered by Office365) for any scenario where you want a fully-functional Outlook experience on the desktop with reliable synchronisation across multiple devices along with other stuff such as push email and webmail.