multipart/mixed, multipart/alternative(1.94.4)

["Soo-Joong, Ghim" <ghimsj @ apwinc . sookmyung . ac . kr>]

Hi, there.
Some subscriber's messages make me get 'BOUNCE' message.

1.
Content-Type: multipart/mixed; boundary="------------OFMAIL265871"

2.
Content-Type: multipart/alternative;
        boundary="----=_NextPart_000_000B_01BE6AE0.92AECB60"

What these mean? And how can I solve this problem?

Anyway, Thank U.