Migrating to "The Cloud" - "The SMTP address has no mailbox associated with it."

Question

We have a front office app that can receive data from Exchange via EWS Push Notifications.  Last week, they moved my mailbox to "The Cloud". Now my mail resources all have "office365.com" in their paths. I'm a very early
adopter, trying to understand how to extend the app to work with this architecture before they move everyone else over.
In the past our app was an EWS 1.1 system talking to Exchange 2007 SP1 servers.  That's the ExchangeVersion I've always had set in EWS. Nobody has ever talked to me about upgrading any of our Exchange boxes.  Impersonation was
not supported in 2007, so we've been using a dedicated service account with full access to specified mailboxes. At the moment, I'd be happy if I could just connect to my own mailbox with my own credentials.
Our old system has 4 CAS servers. You can reach a mailbox through any of them.  For the new mailboxes, we've added two new CAS servers. I'm told these particular servers are Exchange 2010 SP3.  I'm also told the cloud mailboxes are in an Exchange
2013 environment.
I've upgraded my EWS library to 2.2. I've tried ExchangeVersion values for 2007 and 2010 SP2 (there is no value for SP3). I'm not using autodiscover; I just put the name of one of the new CAS servers into the code for now.
My SMTP address is [email protected]. Our internal domain is usa.bigcompany.net, and I commonly login to resources (including my workstation) as USA\smith_j. I was using that ID
for Exchange, but since my mailbox got moved I now login to my mailbox as
[email protected].  We were previously building credentials for EWS with username (smith_j), domain (USA) and password.  Because of the changes, it seems that the right username to pass should
now be [email protected], along with just the password.
The problem is that no matter what I send, the server returns "The SMTP address has no mailbox associated with it."  I've tried a lot of permutations, and for hours that's all I've gotten.  Trying to reduce this to the most basic level,
what should I be doing in order to reach my own mailbox in office365.com, using my own account?  Is there any trick to this?
BTW, if Microsoft is using Exchange 2013 and my CAS server is 2010 SP3 which ExchangeVersion should I be using to do this?

Answer 1

This is very good. Using the office365.com URL, I have been able to create a subscription.  Of course, the office365.com server will not be able to make callbacks through our firewall, but it does clearly show that our CAS server is not getting
the job done.  This is progress!
I asked, and was told we're using Exchange 2010 SP 3 on the CAS because we already had licenses for that.
So I'm back to the question of whether this can be done by an on-premises CAS (or a cluster of them), or if we must go through Microsoft's servers to reach the mailboxes.
And again, thank you!