Email warmup

Warmup is the practice of sending small, conversational volumes of mail from a new (or recently dormant) mailbox to gradually build a positive reputation with mailbox providers like Google, Microsoft, Yahoo and Apple. The goal isn't a one-off score. It's a track record. Mailbox providers reward consistency.

What "warmup" actually means

Modern warmup tools generate this traffic between participating mailboxes: your mailbox sends short, plain-text messages to other warmed mailboxes, those mailboxes open and reply, and the cycle repeats. Done well, the pattern looks like a real human starting to use a new inbox.

Why providers care about it

Inbox placement is reputation-driven. When a brand-new domain or mailbox starts sending dozens of cold messages to strangers, Gmail and Outlook treat it as suspicious by default. They don't know whether you're a small startup or a spammer, so the safest assumption, for them, is spammer.

Warmup gives them signal. Mailboxes that send small volumes, get opens, get replies, and don't trigger complaints look like real human inboxes. Providers loosen the screws as that pattern persists.

What good warmup looks like

Gradual volume growth

Start low and ramp slowly. Warmbly's defaults are 10 emails per day to start, +1 per day, with a ceiling of 40 per day. That ceiling exists on purpose: warmup that climbs to 100/day looks unnatural and starts to look like a bot.

Realistic spacing

Warmup mail should be spread across the working day, with sensible gaps. Sending all 40 messages in a single minute is the loudest possible spam signal.

Conversational behaviour

A real mailbox doesn't only send. It replies. Warmup pools should generate a mix of new sends and replies to existing threads. Warmbly defaults to a 30% reply rate within the pool, tunable per mailbox.

Quality recipients

Warmup recipients should be other warmed, healthy mailboxes: not free trial mailboxes, not throwaways, not lookalikes of catch-all domains. Pool quality matters more than pool size.

Token verification

Every warmup message Warmbly sends carries a verification token that is single-use and bound to its recipient, so warmup mail cannot be replayed or redirected. Nothing is held against a mailbox for the tokens that arrive in its inbox, which anyone can send it. What is held against it is what it does to warmup mail it verifiably received: deleting it, or flagging it as spam. Enough of that, or a poor enough spam-placement or complaint rate, and the mailbox leaves the pool until it requalifies.

A token carried in a header only works if the header survives delivery, and not every provider passes custom headers through. Warmbly records the subject and the provider-assigned message identifier for each send as well, so a warmup email whose header was stripped in transit is still recognized at the recipient rather than being treated as ordinary mail.

What bad warmup looks like

• Synthetic blasting: sending 200 identical messages a day to the same handful of recipients. Looks great on a vendor dashboard. Looks awful to Gmail.
• Throwaway pools: generating recipient mailboxes from free providers on the fly. Providers know what disposable inboxes look like and weight signals from them at zero, or worse.
• Open/click theatre: simulating opens and clicks at unrealistic rates. Apple Mail Privacy already pollutes open rates; pumping the number from a bot makes the entire signal worthless.
• Stopping at "ready": many tools stop warmup the moment campaigns begin. Reputation has to be maintained, not earned once and forgotten.

How long does it take?

Domain warmup commonly takes 3–6 weeks to reach a stable state. New domains take longer than re-warming an existing one. New mailboxes on warmed domains often stabilise within a week or two if the domain reputation is already healthy.

What matters is not finishing warmup: it's running it indefinitely at a low volume in parallel with your cold sending, so any reputation dip is caught and offset before campaigns hit it.

Free pools vs paid pools

Free warmup pools are useful for testing but mix higher-risk participants. Paid pools should be stricter: only vetted, monitored mailboxes participate, and misbehaving senders are quarantined fast.

In Warmbly, free-tier mailboxes never silently appear in the premium pool. Even dedicated-worker customers participate in the premium pool by default. Dedicated infrastructure isn't a reputation shortcut, it's an isolation tool.

When to quarantine a mailbox

Complaints, bounces and tampering are things a mailbox does to other people, so they act early. Spam placement is different: it is a reading of reputation, and warming is how reputation recovers, so pulling a struggling mailbox out of warmup takes away the one thing that would fix it. Placement only ever slows a mailbox down. Our policy:

• Watch: spam placement ≥ 10% over a 20-delivery sample, or complaint rate ≥ 0.03%. Lower volume, increase spacing.
• Throttled: spam placement ≥ 20%. Warmup keeps running at half volume; cold volume is halved too. Lifts by itself once placement recovers.
• Quarantine: complaint rate ≥ 0.10%, or bounce rate ≥ 5%, or repeated tampering with received warmup mail. 7-day removal from the paid pool.
• Block: complaint rate ≥ 0.30%, or bounce rate ≥ 10%. 30-day block.

Which mailbox provider filed a message matters too. Google, Microsoft and Yahoo filter on sender reputation, the same reputation your cold email is judged on, so only their verdicts count. A small mail host runs its own filter with its own rules, so a message in its spam folder is not evidence that the mail is spam, and it is never held against the sender.

Practical checklist

1. Set up SPF, DKIM and DMARC before you start warming.
2. Choose plain text. Save HTML for transactional mail.
3. Use a sub-domain for cold sending (e.g. outreach.acme.com) so any reputation dip is isolated from your transactional domain.
4. Start at 10/day, ramp +1/day, ceiling 40/day.
5. Reply rate inside the pool around 30%.
6. Don't stop warmup when you start campaigns.
7. Watch placement weekly, not opens daily.

Warmup is reputation insurance, not a magic switch. Treat it like a habit your mailbox maintains, not a project you finish.