Hi - I must have missed this one earlier, but to answer your question you can find that out for a specific user by doing a GET on:
 
/vmrest/users/{userid}/mailboxattributes
 
Which should return something like:
 
<MailboxAttributes>
<ByteSize>0</ByteSize>
    <MessageAgingPolicyObjectId>b07c3ae9-81fd-4215-949a-e1547d548d30</MessageAgingPolicyObjectId>
    <Mounted>true</Mounted>
    <NotificationType>0</NotificationType>
    <SendReadReceipts>true</SendReadReceipts>
    <NumMessages>0</NumMessages>
</MailboxAttributes>
 
Hope that helps,
~ Matt