My primary domain holds the single post office
A secondary domain hosts the GWIA and a third domain hosts web access.

Yesterday, I added yet another domain to host a second web access agent and site.
In the process of setting it up, I screwed up the link configurations.

In the link configuration between the primary MTA and the MTA for the GWIA, I put in the IP address of the new webaccess domain's MTA.

After the creation of the new domain and web access, I had noted a huge number of messages like this in the logs:
17:43:24 776 MTP: <domain name>: Returning acknowledge (11)

This was before I was aware of the problem I had created for myself.

When I look at the web console for the new domain, the status page lists 35,000 messages routed and 1878 undeliverable.
I find no messages in any of the domain folders.
I don't think this is version-specific, but I'm not positive about that.

So if this webaccess's MTA starts receiving messages meant for a GWIA that it does not host, what does it do?
Does it reject them or hold them somewhere?

Would the MTA for the primary domain/post office be aware of the problem, or does it only care that the messages have been transferred to the other MTA?