Outgoing mail server probably throttled your mass mailing so they did not spew out all at once. This is a good thing or you might have been black listed as a spammer and then non of your emails would go out.
Getting delisted from an RBL is a major pain in the ass. If this was the case then then delayed emails were a GOOD thing.
http://en.wikipedia.org/wiki/DNSBL#Terminology
Oh and i got the test email about 30 mins ago.
