I'm still Evolution(mail client) user. In reason of KMail2 will be fully migrated onto Akonadi, what means no big changes will came, I decided to switch form Evolution to KMail2.

The problem is: I using homemade script to filter messages - it is very good and never had made me angry.

So: The script is parse content by counting words in my native language and English. Words with are in both languages, but in different meaning, like car are counted twice. Next the script will divide words in native language by sum of words in native with words in English.

Problem is: I can't run external tools form KMail2 to check mails. I can't even use Sieve Script to run aspell or my script.

Plz, help me!

If you run the inbuilt "Anti-Spam Wizard" then look at Settings > Configure Filters, perhaps this gives you an idea of what you need to do to integrate your custom spam filter?

Yes.

But what I should do with exit codes? In evolution I can select "if command returned number, then..."

I have read my homework. I realized the output of external tool is also connected to parent process(KMail).

I suppose, I should only add Lachu-Filter-Spam and detect this tag with other rule.
Correct?

I haven't done a custom filter such as this before, but that does sound correct, yes.

My filter(or KMail2) don't overwrite original e-mail. Why? KMail2 should overwrite original message, when filter returns 0. I send new message to standard output.