I try to import old kde 3.X mail in kmail2. It can import many email. I'm investigate, looking one correct imported email an one not.
The system recognizes the first as an email, the second as txt.
Open the two file with editor the difference is:

1) good email start with:
Return-path: <xxxx@xxxxx.it>

2) wrong email start with:
Resent-From: Nnnn <nnnn.nnnn@nnnn.com>
Resent-To: Rrrr <zzzz@zzzz.zzzzzzzz.it>
Resent-Date: Mon, 31 Jan 2011 12:07:33 +0100
Resent-Message-ID: <201101311207.33113.xxxx@xxxxx.it>
Return-path: <xxxx@xxxxx.it>

If i delete the first 4 lines of wrong email, all is OK.
It's a bug?

If you can, please file a bug report, including the full headers of both types of messages. Details such as from/to/subject and the body of the messages should be obscured but still included (ie. if the header is preset leave it there)

Bug 289379 has been added to the database.
https://bugs.kde.org/show_bug.cgi?id=289379

Moved to Office & Productivity.