Problem with Thunderbird and AmaViS on etch

[Alessandro FAGLIA <a.faglia@farmol.it>]

Dear users,
I have a mailserver (i386) with etch and amavisd-new 2.4.2-6.1. The only 
locales I have generated is en_US ISO-8859-1. On the client side I have 
Thunderbird 1.5.0.9 with french localisation.

Whenever a mail asks for a return receipt (accusé de réception) and 
Thunderbird sends it, amavis mark this as a message with BAD HEADER. 
This is what I read in the quarantined mail header:

X-Amavis-Alert: BAD HEADER Non-encoded 8-bit data (char E9 hex): Subject:
        Accus\351 de r\351ception (...

For the moment I solved the problem by setting D_PASS as default policy 
for banned messages, but I would like to understand why this happens. Is 
this a configuration problem of amavis or what else? I use Thunderbird 
also in other languages (italian, english) and I've never experienced 
such problem.

Any help is appreciated.

Regards,
Alessandro.