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 try the following:
update to the latest version of SendEMailToPage app version 2.x+.