Invalid Header - Non-encoded non-ASCII data (and not UTF-8)

[Egensperger, Fabrice]

Hello,

I'm using Kolab 3.4 with Multi-Domain Support.
I'm not able to receive mail containing non-ASCII data in the mail 
header.

See below error:

********************************************

This nondelivery report was generated by the program amavisd-new at host
domain.net. Our internal reference code for your message is ...

INVALID HEADER

   Non-encoded non-ASCII data (and not UTF-8) (char E9 hex): Subject: 
Secr\x{E9}tariat

********************************************

Does someone already had this issue?
Is there a way to allow utf-8 in mail headers?

Thanks,
Fabrice