Exploring Email Deliverability Services: Are They Legit?

Hey all, I recently joined a startup using a product/agency that claims to improve email deliverability by hosting mailboxes outside of Google and Outlook, smart domain routing, and an improved warm-up process. They promise 100+ emails per day per mailbox and 3-5 mailboxes per domain. I'm just a casual cold outbounder but these numbers go against the numbers I typically see and have used (eg. 20 emails per day per MB, 2 MBs/domain). Have people come across other services like these? Are they legit?