[horde] Showing html content on IMP

[ggarnier@embratel.com.br]

I´m new to Horde and IMP, and I´ve installed Horde 3.0.4 and IMP 4.0.3.
When I get a message in html, sometimes IMP shows it correctly, and
sometimes it shows the html code (it doesn´t interpret the code). I´ve
realized that the html source code is shown when it´s in the message body,
not as an attachment. How can I configure IMP to always show html messages
correctly?
I´ve tried to edit horde/imp/config/prefs.php and change attachment_display
value to ´inline´ or ´both´, but it didn´t work.