The post counts on PM pages (inbox and sent) do not seem to do anything useful.
On the inbox every page displays "0 of N"
On the sent items every pages displays "N of N"
On both the word "unread" follows the "m of n" text ... even though there are no unread items (certainly in the inbox)
Hmmm just as I typed the above, is this intended to indicate how many of all items are unread? In which case the report would be correct for inbox, but quite wrong for sent items. Whilst I do not read event item I send, I have ready many to check back on details of previous PMs.
The functionality is still odd, no matter how one tries to find sense with what's there!