@MC
I've been reading this and would just like to confirm I've understood the problem correctly - You have a mail account set up in Outlook where the outgoing mail server is set as 'relay.plus.net' and both the From: & Reply-To: addresses are the same and is it a domain Hosted by another "ISP" (in this case Google)?
This may be the reason things may be happening differently, unless this was just an unlukcy one-off.
I periodically send to a group of about 50, but the From: & Reply-To: address was @theother ISP, not a domain hosted by them. Unless something's changed since the 20th March, I received an NDR at myname@theotherISP when one of the recipients address was no longer valid (changed address, old address not valid + hadn't told me new one!).
Try a test, setting up another account using your gmail address as the From: & Reply-To: & send a message to the same non-existant address from the gmail one and from the hosted domain one and see what happens. Try sending to a variety of ISP domains. My personal experience over the last couple of years is that here are some ISP's who just dump any mail that 'they' consider to be spam like and no rejects occur and hence no NDR.
The primary point of Bob's post above was to show that using his gmail account as the From: & Reply-To; sending via PN, an NDR was generated OK. I can't confirm (I'll have to dig more) that I had NDR's before IronPort, but I thought I had.
