There were some reports of earlier reports of international characters not working showing up properly in Confluence. However I've just conducted a few tests (March 8th, 2014) with the following email and results:
Screenshot of email sent using Postbox:
Resulting Confluence Page:
This was tested both on Confluence 4.3.7 and 5.4.2
If you are seeing an issue please update to the latest version of AddOn. If you still see an issue please report it to:
support@artemissoftware.freshdesk.com
Please include your version of Confluence and logs would be helpful 
thank you!
Brendan