I don't know what is your "hosted Azure solution" that costs tens of thousands per month (I think your IT guy is ripping you off), but if you were to switch to Office365, for $4 per user per month you could have 25GB mailboxes on servers that you don't have to maintain or patch or backup, without having to stop using your own Active Directory (if you want you can use a managed domain instead). Or for $8 per user per month you would have that plus SharePoint, Lync and the online version of MS-Office, which is at least as good as any non-online version of existing FOSS office suites.
Even with the big plan at $8 that's a $11,200 cost per month for your 1400 users; how you managed to setup an Azure solution that costs tens of thousands of dollars is beyond me, and how you expect to save money by replacing Exchange is also puzzling.
But just for fun let's crunch the numbers in your Azure scenario:
1) 3 Azure servers, plus storage, SQL Azure and bandwidth, that's $1000 per month (or $0.71 per user)
2) Windows Server + Exchange Server licenses: over a 3-year span (typical accounting), that's more or less $150 per month (or $0.10 per user)
This leaves $10,000 per month to pay for IT people, which is not a lot because they get sick, take vacations, etc, so you need at least 3. I'm sure that kind of team can deliver the kind of SLA included with Office 365, which has huge datacenters and a small army of sysadmins.
Show those numbers to your boss and explain to him how switching to Postfix or the free edition of Zimbra (that has no search, calendar or contacts) could save tens of thousands of dollars. I'm sure he will promote you to CIO on the spot. Or if he knows how to count he will see what is the expensive line item in his IT budget and he will outsource it.