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 AddOn version 2.x+.
- Pleas have a look at this page from Atlassian to test if your system otherwise working with international characters - Troubleshooting Character Encodings
If you still see an issue please report it to:
Page lookup error: page "Artemis Software" not found.
If you're experiencing issues please see our Troubleshooting Guide.
Please include your version of Confluence and logs would be helpful 
thank you!
Brendan