Email warmup builds sender reputation before cold outreach starts -- what it actually does, how long it takes, and which tools are worth paying for.
Rees Bayba
Founder, Astra GTM
TL;DR
Email warmup is the process of gradually increasing sending volume from a new domain or mailbox to build sender reputation before starting cold outreach. New domains have no sending history, which makes spam filters suspicious -- they can't distinguish a legitimate business from a newly registered phishing domain. Warmup solves that by creating a record of positive sending behavior before your first real email goes out.
Warmup tools add your mailbox to a network of real inboxes. They send emails on your behalf to other inboxes in the network, which then automatically open, reply to, and rescue from spam. These positive engagement signals -- opens, replies, marked-as-important -- tell Gmail and Outlook that real people want to receive email from your domain.
Warmup addresses four specific problems that new senders face. Knowing exactly what it fixes prevents the mistake of expecting it to solve things it doesn't.
The warmup timeline is not arbitrary. These numbers come from observed behavior across hundreds of domain setups. Going faster triggers spam filters. Going slower wastes time.
Domains that skip warmup and start cold sends immediately typically see 60-80% of emails land in spam within the first week. The 14-day minimum is the floor, not the target.
The differences between warmup tools are real. Network size determines how quickly your reputation builds. Inbox composition (Gmail vs. Outlook split) determines whether warmup helps your Microsoft deliverability. Reply quality affects the strength of the engagement signal.
| Tool | Network Size | Price | Gmail/Outlook Split | Notable Feature |
|---|---|---|---|---|
| Instantly Warmup | 200,000+ accounts | Free with Instantly subscription ($37+/mo) | ~60% Gmail / 40% Outlook | Integrated with Instantly campaigns -- no separate setup |
| Mailwarm | ~10,000 accounts | $29/mo | ~70% Gmail / 30% Outlook | Simple setup, good for beginners |
| Warmbox | ~35,000 accounts | $15-99/mo (per mailbox pricing) | ~65% Gmail / 35% Outlook | Lowest cost entry point, per-mailbox billing |
| Warmy.io | ~50,000 accounts | $49+/mo | ~55% Gmail / 45% Outlook | Best Microsoft/Outlook network representation |
If your prospect list skews heavily toward Microsoft 365 users (enterprise, law, finance, healthcare), Warmy.io's higher Outlook network proportion is worth the premium. For general B2B cold email with mixed inbox providers, Instantly's bundled warmup is the easiest path if you're already using Instantly for campaigns.
Warmup is a targeted solution for one problem. Treating it as a general deliverability fix is the most common mistake. These problems require different solutions.
Ongoing maintenance. This is the most common misconception about warmup. Most operators run warmup for 2-3 weeks, hit their target reputation, and then turn it off when campaigns go live. Sender reputation decays when you stop sending positive engagement signals. Warmup running at 20-30 emails per day counterbalances the neutral or negative signals from cold outreach. Turning it off while sending campaigns is like removing the foundation while adding floors.
How long does warmup take?
14-21 days minimum. Start at 3-5 emails per day and increase by 2 per day each week. Most operators run a full 3 weeks before sending cold campaigns. Some see good inbox placement after 10 days, but 14 days is the minimum to avoid triggering spam filters on your first cold sends.
Which warmup tool should I use?
If you use Instantly for campaigns, use Instantly's built-in warmup -- it's included in your subscription and integrated. If your prospects skew toward Microsoft 365 users, Warmy.io has the highest Microsoft/Outlook network representation. For the lowest cost entry, Warmbox starts at $15/mo per mailbox. The tool matters less than using one consistently and keeping it running.
Can I send cold emails during warmup?
Not in the first 14 days. After day 14, you can start with a very small batch (20-50 emails) to test before scaling. Run warmup and campaign sends simultaneously from day 15 onward. The warmup engagement signals help offset the lower engagement rates from cold prospects who don't open.
What happens if I skip warmup?
Your emails will land in spam immediately on most mailboxes. New domains with no sending history look identical to freshly registered phishing domains to spam filters. Most senders who skip warmup see 60-80% of emails in spam within the first week, and the damage to their domain reputation takes 4-6 weeks to repair -- far longer than the warmup they skipped.
Does warmup work differently for Gmail vs. Outlook?
Yes. Warmup builds reputation on both platforms, but Microsoft is more sensitive. For good Outlook inbox rates, you need a warmup tool with significant Microsoft/Outlook accounts in its network (aim for 30%+). Gmail reputation builds faster. If your warmup tool is Gmail-heavy, your Google inbox placement will improve but your Outlook deliverability won't improve as much.
How do I know if warmup is working?
Three signals: your warmup dashboard shows emails landing in Primary inbox (not Spam), Google Postmaster Tools shows your domain reputation moving to 'Medium' or 'High,' and when you send test emails to your own accounts they hit the inbox. If after 14 days your warmup emails are still landing in Junk on test accounts, check that your DNS is configured correctly -- bad SPF/DKIM/DMARC will prevent warmup from working.
We implement these systems end-to-end. First sends within 14 days.