Update: For some reason I couldn't reply to your response kstrauser so here is my reply: That works if and when their is trust. First you have trust the buyer isn't going to abuse the system and wreck the IP [reputation] and you have to trust the buyer isn't giving you stolen info. Obviously things one can work through but again, it involves trust. Also the provider has to have warmed up IPs to give out. Which, having warmed up IPs would actually be a great valued add upsell for those that need them!
Update 2: Thank you FfejL and symfoniq for explaining it better.
I'm not saying this to advertise but to offer a suggestion: call someone and ask for help. This isn't an unusual need at all and any reputable provider will be quick to help.
Short version: you don't have to "warm up an IP" if you do a little advance homework.
If Google (or any other ISP) suddenly sees an big spike in email from @example.com on an IP that @example.com hasn't used previously, Google is much more likely to mark those emails as Spam.
So senders need to 'warm up' an IP by sending a small amount of email first, usually for a few days at least.
So if you're a SendGrid customer expecting to send 2,000,000 emails today, you can't just switch to a new ESP and send those same emails. Spam rates will go through the roof.