According to the message flow charts (link) the GWIA is actually an agent of the MTA, not the POA. SMTP Message flow hits the GWIA, routes to the MTA, which in turn routes to to the POA.
AS for simulating load of the POA, it really depends on what kind of load you want to simulate. There are several kinds:
- Users Sending Mail Normally. This will require entering the messages arriving over TCP from clients, entering messages into the per-user mailbox databases, filing any Attachmentss, and sending messages as needed to the MTA.
- Users Receiving Mail Normally. This mail inbound from the MTA, entering mail databases, and filing any attachments.
The first bullet point is nearly impossible to simulate since it requires an active connection from the client. To simulate that you'll have to write a TCP program that can simulate GW client load, and I don't know of any tools to do that.
The second point is easier, since you can drop files into the MTA's /mslocal/$postx/0-7 directory, where the MTA will transfer it to the POA for filing. MTA directory-layout guide. It'll put load on the MTA, of course, but it's the easiest way to pound a POA.
Unfortunately, I no longer remember the file format used by the MTA.