A brand new mailbox has no history. If it suddenly sends a hundred cold emails, inbox providers have no reason to trust it. Warm-up tools try to build that history first. They help, and they have limits.
What warm-up does
A warm-up tool connects to your mailbox and exchanges emails with a network of other inboxes. Those inboxes open the emails, reply to some and move them out of spam if needed. The volume grows a little each day.
Apollo's help article on warm-up describes it this way: the emails simulate real traffic, and that activity signals to mailbox providers that your mail can be trusted. Apollo's warm-up runs through a third-party provider, Warmbox.
What it cannot do
Warm-up is one part of getting into the inbox. It cannot:
- Fix a broken setup. If your domain is missing SPF, DKIM or DMARC, warm-up does not add them.
- Fix a bad list. Emails to old or guessed addresses bounce. Google's guidelines say to cut your volume when messages start to bounce.
- Change what real readers do. Google and Yahoo watch how often people mark your mail as spam. Warm-up emails do not change that number for your real outreach.
- Rescue a damaged reputation on its own. Apollo's help article says warm-up may not be enough to recover, and that you may need a mix of changes.
The basics that matter more
Before you pay for anything, check these. They come from Google's email sender guidelines and Yahoo's sender requirements.
- Set up SPF or DKIM for your sending domain. Senders of more than 5,000 messages a day to Gmail need SPF, DKIM and DMARC.
- Keep spam complaints low. Google and Yahoo both say below 0.3%, and Google's guidelines aim for under 0.1%.
- Raise your volume slowly. Google advises starting low, sending to engaged people, and avoiding sudden spikes.
- For marketing messages sent in bulk, offer one-click unsubscribe and a visible unsubscribe link.
In plain words:
- SPF lists the servers allowed to send email for your domain.
- DKIM adds a signature that proves an email came from you.
- DMARC tells receiving servers what to do with email that fails those checks.
Your email provider gives you the exact records to add. There is a short guide in how to set up a business email address.
Warm-up built into sending tools
Many cold email tools include warm-up. As of October 2026:
- Instantly lists unlimited warm-up on its Growth and Hypergrowth outreach plans. Its warm-up settings guide suggests about 10 warm-up emails a day for a new account, rising slowly.
- Smartlead includes warm-up on its plans, with a different warm-up pool per plan.
- lemlist includes its warm-up tool, lemwarm, in all plans.
- Apollo says paid plans usually include warm-up for one mailbox per seat. Extra mailboxes, and mailboxes on the free plan, use 200 credits every 30 days.
If you already pay for one of these, start there.
Standalone warm-up tools
If your sending tool has no warm-up, or you send from your own inbox, a separate tool can help. As of October 2026:
- Warmbox has a Solo plan at $19 a month on monthly billing for one inbox and up to 50 warm-up emails a day. It is $15 a month on yearly billing.
- MailReach prices warm-up per mailbox, listed at $19.50 a mailbox a month, and includes spam test credits with each warm-up plan.
How long to warm up
Apollo recommends at least two weeks. Warmbox says first results usually take 15 to 30 days. Neither is a promise, since every domain starts in a different place.
After warm-up, start real sending slowly. A few emails a day per mailbox at first, to people likely to care, then more over the following weeks.
Then write emails people want to answer
Warm-up imitates people reading and replying. Real readers do that when you write to the right person with a short, specific message. See how long a cold email should be and why cold emails sound like a robot.
Check the law too. In the US, the FTC's CAN-SPAM guide requires a working way to opt out, honoured within 10 business days.