A bit more testing:
I changed the password so the client doesn't know it.
Just viewing in his account the emails that come in now.
If I send an email from an account not on the same server then it sometimes increments the number of unread emails.
If I send to the account using and email account on the same server it doesn't register as an unread message.
In all cases the email list view doesn't show any in bold, so I can't tell which are unread.
So, I sent 1 email from outside the mail server, this incremented the number of unread messages, then i sent one from inside the same server, this had the action of decrementing the unread number!
Now when i send, inside or out, the number doesn't seem to change.
And has just closed the browser tab.
After going back in, there are no unread messages showing, either number or in the email list.
There definitely seems to be something wrong with this account?
It also only affects his Inbox.
I created a sub folder called Test and added content filtering to the test email account i was sending from and the email gets put into the test folder, is marked as unread and is in bold so it looks like its been unread.
So looks like an inbox issue?