I bought sparkpost and they have some problem working with interspire which i got to know later. I asked them to give me time to fix things as i use interspire and you are the only one who need a patch to be added. Neither they refunded and keep billing me for no use. and nor they replied to my request. Handle with care.
After going through send grid, mailjet, now happiest with mandrillapp
Although there is no telephone support, the support we have received has been excellent.
The product is simply awesome, its worth the money!