I reinstalled the whole Horde Stack and now it seems to work.... sorry 
for the noise.

When trying to send a mail, i can enter whatever recipient I want - I 
always get the message "Invalid e-mail address (<address>)." No metter 
what address I've entered.

I've already checked lib/Compose.php Function _prepSendMessageEncode 
where the error is raised.

The passed Horde_Mail_Rfc822_List looks like this:
http://pastebin.com/swfsQtax

I also tried to work around this manually to bypass the foreach loop with:
$out[] = $val->__get('bare_address');
continue;

This brought me a bit further, but now it failes outside the function 
with "Address is missing domain".

I've no idea what to do further here but as I said - I can no longer 
send any email.....