What is the source of the outages? Poor management on the part of your friend or just downtime from the ISP? If the Exchange Server outages are from ISP downtime, then I'd just switch hosts. E-mail outsourcing can be a bit pricey; you'll save money by running your operation off a VPS from a reliable host.
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The other option is to keep your own email hosting, and use someone else to act as a secondary mx for if you go down. Then when you come back up, the other server will send to you on its next queue run.
I'm not sure whether they can handle the kind of requirements you have, but I use Webmail at http://webmail.us for our email. I switched to them because our previous host, GoDaddy (no laughter please ) didn't offer IMAP.
Performance is excellent, and their spam filters are top-notch.
Host/Executive Producer, The Paracast, www.theparacast.com
I do not represent the hosting industry!