Getting into the inbox is mostly about history, and history takes time to build.
Mail providers decide what to do with a message largely on the basis of what they have seen from that sender before. A domain with no history is treated with suspicion, which is reasonable, because that is what a spammer looks like on day one.
So sending capacity has to be built up gradually. Volume that ramps too quickly gets filtered, and once a sender is being filtered it takes considerably longer to recover than it took to cause.
If a vendor offers you full volume in the first week, either they are about to burn new infrastructure on your campaign or they are sending from something already used heavily by somebody else. Neither is good for your reply rate.
It also matters whose domain is being spent. We buy and maintain the sending domains, so if a campaign goes badly the damage is ours to repair and your brand keeps reaching your own customers.
The data you are buying, the campaigns going out, what your reps are doing and what the CRM says happened to all of it. You get the findings in writing and you keep them whether or not you work with us. There is a quote at the end and no obligation to take it. We would rather tell you your current setup is fine than sell you something on top of it.