OK...this seems to be the issue. Our Sympa admin messages had the following header field which was empty.
Content-Transfer-Encoding:
We removed it from a test message and it delivered correctly to the device. Something to do with localization...our Sympa Administrator is going to fix it I think. Not sure it's technically a bug in Zimbra, may just be a misconfiguration in Sympa, but I did enter it in bugzilla just in case...
Access Denied (hidden because it has some mail header info in it)
Thanks,
Matt