And thats the problem. The preferences 'sending_charset' is not added 
into the db-table. It works when I add it manually into the db.

Then set the 'sending_charset' preference to an appropriate value.

Yes these are Turkish characters. But the problem also happens with 
other languages:

Ì c3 8c    LATIN CAPITAL LETTER I WITH GRAVE

Р   c3 90    LATIN CAPITAL LETTER ETH
å    c3 a5    LATIN SMALL LETTER A WITH RING ABOVE
æ    c3 a6    LATIN SMALL LETTER AE
ë    c3 ab    LATIN SMALL LETTER E WITH DIAERESIS
ï    c3 af    LATIN SMALL LETTER I WITH DIAERESIS
?    d8 8f    ARABIC SIGN MISRA
?    d8 a8    ARABIC LETTER BEH
?       d8 b9    ARABIC LETTER AIN

It used to work with Horde 3 and PHP5. The E-Mail encoding of IMP 
generated mails are always ISO-8859-1
Content-Type: text/plain; charset=ISO-8859-1; format=flowed; DelSp=Yes

Emails sent with other clients that set encoding to UTF8 are displayed 
fine with IMP, even with turkish. The PHP 5 bug with Turkish is 
related to lowercasing and uppercasing characters.

Are these turkish characters?  If so, this is a PHP 5 issue.

I just see that the characters I added to the below comment are also 
substituted

UTF8-Chars (like ??, ?I, i?, ??) are destroyed when mail is composed 
with imp (traditional, dynamic).
I see that any mail send out of IMP is composed with:

Content-Type: text/plain; charset=ISO-8859-1; format=flowed; DelSp=Yes

I would expect it to be UTF8