EBS R12 XML Gateway: How to define the mail server for ECX_INBOUND?

I'm trying to setup up the XML Gateway in order to receive E-Invoices by E-Mail.

When testing with the ../webservices/ECXOTAInbound test site, it seems to work fine:

• the test item is queued in ECX_INBOUND and
• the dequeued and processed (resulting in other errors, which are probalby another story)

But my question is, where do I define the mailbox that should be used to fetch the invoice mails and queue them in ECX_INBOUND?

As far as I have seen, this should be covered by the OTA. When testing the OTA with …/OA_HTML/US/ECXOTAPing.htm, the test screen to enter the testdata pops up, but hitting 'Ping' with test data gives always a 404 error - page not found.