[imp] Configure UTF-8 viewing in Horde Imp

[William Krick]

My host recently upgraded my Plesk installation from 8.6 to 10.2 and  
in the process my installation of Horde Imp webmail got upgraded.

Something now appears to be configured incorrectly as I often get this  
warning at the top of my emails...


This message was written in a character set (utf-8) other than your own.
If it is not displayed correctly, click here to open it in a new window.


My server is running a version of RedHat linux.
When I run this command...

locale -a

...there are three results for en_US...

en_US
en_US.iso885915
en_US.utf8

...so I think the server is configured correctly.


Is there somewhere I need to configure Horde/Imp so that it can  
display UTF-8 when I VIEW (not send) messages?