Emails get maked as unread after archiving

This is most likely seen if Outlook has Cached Exchange Mode turned on. If it does, this issue is caused when a message that was unread, is marked as read, while offline. The message is marked as read, but only on the local OST and it has not replicated with Exchange yet, to synchronize the settings.

Therefore, when Enterprise Vault makes its pass during an archived run, it archives the original message from Exchange, which is still "unread" and archives it using that attribute. As a result, you will see an unread archived message within your local mailbox

Refer the following document http://support.veritas.com/docs/317734

I agree with Rahul.  If you're seeing this under certain conditions, then the technote hits the mark (and I worked on the associated Etrack).  If you're seeing it outside of the following sort of condition, then it's worth investigating further ..

Conditions you'll see this under :-

Highlight 100 unread items
Right click,  mark them as read
Click the Store in Vault button in Outlook

*Some* of the items won't stay read, they'll flip back to unread.

I need to confirm with the user, but I'm pretty sure that the emails in question had been read some days earlier, and although the user is using Exchange cached mode the computer is always on and connected to the network.

Therefore I can't see this is an issue with the OST not sync'ing in a timely fashion, unless for some reason its not updating with Exchange correctly.

Something else of note in reference to the support document, the emails weren't archived manually but were done as an automated process during the Archiving Task (after I'd enabled the users mailbox for archiving).

I've definitely not seen that issue before then, I'm afraid.