Well it seems Habbo are rolling out this update piece by piece. I was actually away when this update happened (hence the reason that it took so long to add).

Today Habbo UK went down for a maintenance break and when it came back up users were able to send and receive their mini-mail (not to be confused with email) inside the client. But there is a bug, it seems that browsers such as Chrome, Safari and any version of Firefox before 3.0.13 will not be able to show it on top of the client but instead on a loaded page beside the client. But anyone using browsers like IE or Firefox 3.5+ will be able to view it as it is meant to be.

I will try and get screenshots soon.