We have a PMTA and a interspire working for almost one year now.
All works perfectly but there must be any issue coz out newsletter sent about 2 or 3h ago still didn't send 1.000 emails of 8.000.
We have 8 dedicated IPs perfectly configured, IPs with rotation and rDNS, etc, until now we didn't face any issues, before it was faster.
Emails are being sent locally from interspire, we just send one or two campaigns per month to about 8.500 subscribers.
We need a very expert guru and affordable that will see the problem in seconds and will get a fee per hour.
Also, since we're a IT provider, if all goes good we will do more things with you. Always looking for long terms relations.