Pre-warmed inboxes are basically the marketing equivalent of buying a used car from a stranger who says "don't worry, it's fine." Yeah, that could work. Or you could inherit a pile of spam complaints and a domain that's already been flagged by Google. You just don't know what that inbox did before it landed in your hands.
The big problem? Most providers treat these things like rental scooters - someone warms them up, runs a half-arsed campaign, trashes the reputation, then hands them back. And you're left cleaning up the mess without even knowing the damage exists. If the provider doesn't track prior campaign history (and plenty don't), you're flying blind.
So before you throw money at any of those three names floating around, ask these: do you get full ownership of the Google Workspace org, or are you just a tenant on someone else's infrastructure? Can they show you proof of warmup methodology and what kind of pool diversity they used? And what happens to your sending history if you decide to cancel next month? Most will dodge those questions.
Honestly, for the scale you're talking about, just set up your own Google Workspace on your own domains and run a proper 3-4 week warmup. The cost difference is pennies. you own everything, no inherited baggage. Everyone I know who got burned by pre-warmed inboxes wishes they'd just done the legwork themselves. saves you the headache of finding out your "pristine" inbox was actually used to sell dodgy supplements to angry Gmail users.
If you still insist on buying, run independent placement tests before you launch a single campaign. Don't trust the provider's metrics - they'll tell you your deliverability is great right up until your bounce rate skyrockets. Saves tears later.
