Hello, I have next problem:

Clients receive messages in Outlook 2010 without text in message field. The messages contain Japanese characters in subject and body. Despite subject with Japanese characters is correctly displayed, there is no any text in body of the messages. But if clients try to open the messages in web interface mail, messages are correctly displayed with text in body. Also, same messages correctly displays in Mozilla Thunderbird.

Could you tip what could be wrong with Outlook in such situation?

looks like an encoding issue. try opening the message, and click Actions > Other Actions > Encodig, change it to UTF-8. If the email shows properly after that, click Save button to save the changes to the message.