Is there anyway to get the same "MessageId" you can get in Exchange EWS when using ActiveSync?
I thought this was an Exchange way to identify each message uniquely, but I can't seem to find a way to retrieve it using ActiveSync.
EDIT: I've got 2 applications, one that stores info using ActiveSync, and one that stores info using EWS, and I want them to be able to work separately on the same message.... To do this, I was hoping to use the EWS MessageId, which seems to be a GUID type identifier for each individual message. (Note: This doesn't appear to be the same Message-ID as is found in email headers).
Sadly, you're mostly out of luck.
ActiveSync is not an integration protocol, it's a mobile synchronization protocol designed for low-bandwidth communication devices like smart phones. A lot of capabilities in EWS will not exist in EAS.
Long-term message identification and correlation isn't as important for mobile devices. They simply get told what messages are in each folder, and allow the user to manipulate them. At any time the Exchange server may tell its EAS-connected clients to "re-sync" which causes them to forget the messages they have on the device and pull them cleanly from the server. That happens a lot with EAS, sometimes a couple of times an hour, depending on what is happening with that mailbox. For example, deleting a folder via Outlook causes a FolderSync to happen, and that forces connected devices to cleanly re-sync again.
Therefore EAS appears to have left behind the notion of GUIDs or other long term IDs for messages. Instead, the server will assign ephemeral IDs that are valid only until the next big resync is forced (which could happen at any time). You'll probably see Exchange give very simple IDs like 7:45 (which means message ID 45 within folder 7, IIRC). However after a resync that might have the number 7:32 (if the user deletes other messages in that folder) or something like 4:22 (if the message gets moved to another folder entirely).
Other EAS servers like Zimbra, Kerio or Notes Traveler might assign GUIDs, but from memory this is how Exchange behaves. Your only option might be to put a hidden correlation ID of your own into the body or subject of messages you're interested in. That will allow you to track the lifecycle of the items you're interested in, at the expense of some odd stuff being visible to users in their message contents.