Redmine

Environment:
Redmine version 2.5.1.stable.13139
Ruby version 1.9.2-p0 (2010-08-18) [x86_64-linux]
Rails version 3.2.18
Environment production
Database adapter Mysql2
SCM:
Subversion 1.6.12
Mercurial 1.6.4
Git 1.7.2.5
Filesystem
Redmine plugins:
redmine_code_review 0.5.0
redmine_drafts 0.2.0
redmine_knowledgebase 3.0.4
redmine_lightbox 0.0.1
redmine_stealth 0.6.0
redmine_wiki_extensions 0.6.4

Hello,

I am not able to solve the message "invalid byte sequence in US-ASCII"

Completed 500 Internal Server Error in 26.7ms
ArgumentError (invalid byte sequence in US-ASCII):
app/models/mail_handler.rb:46:in `receive'
app/controllers/mail_handler_controller.rb:25:in `index'

Following Cases:

writing E-Mails from Thunderbird, everything is fine.
From Outlook it will stay in MDA temporary failure. Command output: Failed to contact your Redmine server (500).

written languages: german and english
utf-8,iso-8859-1, iso-8859-15

Perhaps someone can give me some simple tips or links for reading.
Iam not an ror dev.

Thx