Most probably that problematic message has HTML format and contains the following symbols, for example - "…"
When Atmail generates XML and sends it to the browser it converts HTML symbol … to an invalid Unicode sequence.
As far as I know, developers have tried to use another browser (Opera) and were able to open such message successfully.
Our developers are aware of this issue, they are going to contact Atmail developers as well. This will be fixed in the nearest future.
Meanwhile please use one of the browsers that ignores the invalid symbols in XML as a workaround.
This issue affects Internet Explorer 7, 8 and Firefox 3.6 (according to our knowledge at least).