Plaintext notification emails are sent as mime multipart ...

I have problem with setting up email notification profile - I have set email to be sent as "plain text" in notification profile, but notification emails are still sent as mime multipart. What am I doing wrong ?

I am using OpManager on CentOS 4.4. I have tried to install it on Windows 2003 server too, but the result is the same - plaintext notification emails are sent as multipart. I have also tried different mail servers (postfix, sendmail, exchange), but there is still the same problem. On the other hand when I send plaintext email from PC (Outlook) it will be received as plaintext, therefore I assume that problem is in OpManager.