The example message you provided is not in UTF-8, hence the error seen. (UTF-8 has certain constraints that allow programs to do limited checks for conformance, “valid byte sequences”.)
The message has a content-type header, but it does not specify charset on that header, that normally would make figuring out the character encoding difficult, but there’s a strong hint in the subject. Following old conventions, the headers are 7-bit safe here, and the GB2312 tells us the charset, and the following B tells us the transfer encoding. (The format is MIME encoded word.)
$ echo sKLA77DNsM274dSx0MXPorLJvK/I7bz+ | mmencode -u | iconv -f GB2312 -t utf-8
阿里巴巴会员信息采集软件
With charset hint, I can see the first line of the message goes something like this:
一锟斤拷锟斤拷锟斤拷锟斤拷桶突锟皆憋拷