I set the font in "XamRichTextEditor_Loaded" to "Arial" Size "9".

private void XamRichTextEditor_Loaded(object sender, System.Windows.RoutedEventArgs e)
{
	XamRichTextEditor rte = sender as XamRichTextEditor;
 
	CharacterSettings cs = new CharacterSettings();
	cs.FontSize = new Extent(9, ExtentUnitType.Points);
	cs.Bold = false;
	cs.Italics = false;
 
	FontSettings fs = new FontSettings();
	fs.Ascii = "Arial";
 
	cs.FontSettings = fs;
 
	string error = "ERROR";
	rte.Document.ApplyCharacterSettings(DocumentSpan.All, cs, out error);
 
	RtfBoxBody.Selection.Text = "Dies ä ist ö ein ü Test";
}

Now in the RichTextBox is the Text "Dies ä ist ö ein ü Test". When I export the text to Outlook, it is displayed in "Arial" but the "ä, ö and ü" is displayed in "Curier".