I've tried both methods:
charset doesn't change the result at all (i get the same mistakes)
while using QuotedPrint i receive the message encoded( like this:'=88=DD=88=AE=88=A8=88=BE=88=BC')
Comment on Re: Re: letters with diacritics when sending mail
This might mean that the recipient's mail reader (MUA) doesn't handle QuotedPrintable correctly, or that you aren't sending the correct headers for it to recognize it.
Sending in 8bit is not recommended, since you might have different encodings on the sender side and the recipient side.