I don't interspire but I've heard about it.
you pretty much will find yourself moving around with hosts, for only 20-30k a day you might last quite some time from box to box...probably going to be changing IPs every couple of days though...depending on whether your mails will have links in them and whatnot.
you could try hivelocity, xlhost, krypt
all 3 are data centers that I've blasted off from. I've easily sent out 10-20 million mails from each of those companies in the last 2 months but I'm compliant so no terminations or anything like that. XLHOST WILL hit you up if you start blacklisting IPs too quickly, but with 20-30k unless your data truly sucks you probably won't have that issue.
bump this thread with interspire stats, I'm interested, it's an MTA right?