Thanks for the advice. I ended up making due with what I had. I'm on a
shared server with shell access, but the htmlcheck function kept crashing
with an out of memory error. Even tried the mem switch to no avail. The
issue was Google's poor wrap of the emails from them. The emails that kept
showing as a problem were ones with a incomplete tag for </table>. They were
trying to wrap the entire body content in a table, but failed to close the
table properly using </table without the closing >.
New issue on a mailman mail list archive. I'll start a new thread.