A new sending domain has no reputation, so Gmail and Microsoft treat it with suspicion. Send two hundred cold emails from it on day one and most of them land in spam. Warmup fixes that by ramping volume slowly while other mailboxes open your messages, reply to them, and pull them out of spam, until the receiving providers decide you look like a normal person rather than a new bulk sender.
Every tool below does the same core job through a pool of real mailboxes that email each other. What separates them is whether warmup comes bundled with the platform you already send from, or whether you are paying for it standalone on top of everything else.
We are not on this list. Outsolvi is a tracker, not a warmup tool. There is a section at the end about the one part of this we do have an opinion on: warmup makes fake opens on purpose, and that has consequences for every open number you look at afterwards.
If you already send from Instantly or Smartlead, use the warmup that comes with it and stop reading. Paying twice for the same function is the most common mistake here. If you send from your own mailbox, or from a tool with no warmup built in, look at MailReach first, because it tests inbox placement as well as warming, so you can see whether it worked. Warmup Inbox is the simplest option if you want one job done and nothing else.
How we ranked these tools
Ranked on three things: whether warmup is bundled with a sending platform or billed separately, whether the pool is large enough to matter, and whether you get inbox-placement testing alongside the warming itself. Deliberately no prices. Warmup pricing changes often enough that a figure published today is a liability in six months, so each entry describes how it is charged instead. Check the vendor's own page before you buy.