The message was quarantined because there was content in the message which could not be scanned. Usually this means one of a couple of things:
1. There was some sort of encoding issue on the message. It said it was encoded in Quoted-Printable, but something was incorrect about that; the message is missing a MIME-parts breakpoint; etc.
2. The message has an attachment which appears to be corrupt. For instance, there's a document attached which says it's a Word document (based upon magic number, not the filename), but the format of the file, while very similar, does not match that of a Word doc.
3. The MEG is configured to protect itself from segvs, and received a file which caused segvs, and thus quarantined the message per policy.