How can I stop Exchange dropping AutoReply emails?

I've setup a simple rule on one of our mailboxes which replies to every email it receives using the "have server reply using x" rule.

This is just to send a confirmation of receipt email to anyone who sends something in.

The rule also ignores our inteal domains (using "except with x in the senders address) and I can see it working fine until it actually hit's the Exchange back-end. If I review the Message Tracking Log I can see that any "AutoReplies" are being dropped with the following message:

250 2.1.5 RESOLVER.MSGTYPE.AR; handled AutoReply addressed to exteal recipient

The SourceContext also shows:

BlockARToExtealUser

I can't find much with regards to tuing this off, can anyone shed any light?

Note that if I remove the "except with x in the senders address" it does in fact deliver the auto response to an inteal mailbox without issue. This only seems to affect exteal email