How to approach email for multiple businesses?

What are the best solutions for this situation:

    - You are running more than one company.
    - You might have to respond to or monitor more 
      than one email address per company.
    - You have more than one person per company with similar 
      needs
    - You want a unified portal from which a person can manage 
      all email for all businesses
    - You don't want a massive list of emails all mashed
      into a single inbox
    - You want to segregate email hosting from web and 
      DB hosting
    - You want online access to all email rather than limited
      desktop-only access

I had a look at Office 365 and Gmail. I can't say either one excited me. At first it looked like Office 365 could cover most of these bases. I have to say, Microsoft needs a lobotomy.

My current solution is to run multiple instances of Office on the desktop. Each one has it's own PSD file and they are, for all intents and purposes, fully segregated. For those of use who have to have visibility into multiple accounts (info@, support@, etc.) it's a very simple matter of adding those accounts and having those emails sorted to their own inboxes. So you can have full visibility into all the email you need for one of the businesses within an isolated application window. Need to do email for the other business? Pop over to the other instance of Outlook and you've switched environments.

As much as I hate to suggest this, I am now thinking that running a Linode instance as a pure email server might be the ticket. I could then have people log on to their webmail. One could have one company per tab and context switch very easily. Other Linodes would host web and DB, just as planned.

I am hoping I am not aware of a really cool unified solution that can deal with this elegantly. Looking forward to hearing what HN members might suggest.