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Email warmup: how long to warm up a new mailbox before cold email

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Plan on two to three weeks of email warmup before a new mailbox sends its first cold email. That is common practice, not a provider rule. Two weeks is the usual floor, and a third week helps when the domain is brand new or the warmup score has not settled. After that, cold volume should still climb slowly for one to three weeks, depending on your daily cap, and warmup usually keeps running for as long as the mailbox is in use.

Time alone does not make a mailbox ready. You also want a stable warmup score, few bounces, almost no spam complaints, and nothing worrying in Google Postmaster Tools. This article explains each signal and what to do when one turns bad.

What email warmup is, and why new mailboxes need it

Mailbox providers such as Gmail, Outlook and Yahoo decide where your email lands partly based on your sending history. A mailbox created last week has none.

Warmup builds that history gradually. Usually it is automated: a warmup service connects your mailbox to a network of other mailboxes that exchange ordinary-looking messages, open them, reply to some, and move any that land in spam back to the inbox. The aim is for providers to see steady positive engagement.

Google states the principle in its email sender guidelines: "Start with a low sending volume to engaged users, and slowly increase the volume over time." The same page asks senders to send at a consistent rate and avoid sudden spikes.

This is also why cold email usually goes out from separate sending domains, not your main company domain. Sizing is covered in how many mailboxes and sending domains you need for cold email.

How long to warm up email before the first campaign

No mailbox provider publishes an official number. Google says to ramp up gradually but gives no timeline, and other advice varies widely.

A common practice range is two to three weeks of warmup only, with no campaign email at all. In practice, two weeks is a common minimum: by then the mailbox has some history and the score has had time to settle. The third week absorbs startup problems, such as a slow DNS change.

Use two conditions together, not one:

  1. At least two weeks of warmup behind the mailbox.
  2. A warmup score that has stayed above roughly 90% for several days.

A high score after four days means little: with so few warmup emails sent, one message in spam swings the percentage a lot.

Why to buy sending domains a few weeks ahead

No mailbox provider publishes domain age as a filtering criterion. Still, a common practice is to buy sending domains a few weeks ahead, for practical reasons: each new domain needs its DNS records set up and verified, including SPF, DKIM and DMARC (the records that prove your mail is authentic), then two to three weeks of warmup. A spare domain warming in the background can replace one you retire.

Ramp up volume gradually

Once warmup is done, do not jump to full volume. A common practice schedule for one new mailbox, with typical numbers rather than provider rules:

  • Week 1: warmup only, often starting around 10 warmup emails a day and rising by one or two each day. No campaign emails, whatever the score shows.
  • Week 2: warmup only, still rising. By the end of the week the score should be above 90%.
  • Week 3: if the score already holds above 90% and the domain is not brand new, send the first campaign emails, starting at about 2 a day and adding about 2 more each day. Otherwise, keep week 3 for warmup only.
  • Week 4: keep adding a little each day until you reach your planned daily cap. If week 3 was warmup only, start campaigns here, at the same slow pace.
  • Week 5 onward: steady state once the cap is reached, with warmup still on.

Follow-ups count toward the daily cap, so a three-email sequence fills it faster than you might expect.

How to read a mailbox's health

What the warmup score tells you

Most warmup services show a percentage: the share of warmup emails that reached the inbox rather than spam, usually over the last week or so. If a mailbox sent 80 warmup emails and 79 reached the inbox, the score is 98.75%.

Common practice reads it like this:

  • Above 90%: healthy, if the mailbox also has two weeks of warmup.
  • 80 to 90%: watch closely and reduce campaign volume.
  • 60 to 80%: something is wrong. Pause campaigns and investigate.
  • Below 60%: stop sending from this mailbox.

The score is measured on the warmup network's own friendly mailboxes, so it can look better than your real inbox placement with prospects. Its value is in spotting a drop early.

Bounces come first

When a warmup score falls, look at bounces first. The cause is often the list, and an email verification step, which checks addresses before you send, is the usual fix. Google's sender guidelines say that when messages start bouncing or being deferred (temporarily refused), you should reduce sending volume until the error rate drops, then increase slowly again. Thresholds and list hygiene are covered in cold email deliverability.

Spam complaints and Postmaster Tools

Spam complaints are the one signal with an official limit: Gmail and Yahoo ask senders to stay under 0.3%, and Google recommends under 0.1%. Set up Google Postmaster Tools on every sending domain from day one, even if its dashboards stay empty at low volume. A low spam rate there is not proof of health either: it only counts mail that reached the inbox. How to read the spam rate, the reputation levels and blocklists is covered in cold email deliverability: SPF, DKIM, DMARC and the bulk sender rules.

What a drop in replies can signal

If replies drop sharply on a campaign whose list and copy did not change, suspect deliverability before the message: check the warmup score, bounces, Postmaster Tools and blocklists.

How often to check

Review the warmup score and bounces daily while a mailbox is new or sending, and Postmaster Tools and blocklists weekly. Catching a drop early usually makes recovery easier.

What to do when health drops

Act in steps matched to how bad the signal is. These levels are common practice, not provider rules.

Mild: score between 80 and 90%. Halve campaign volume on that mailbox and confirm warmup is still running. Once the score stays above 90% for about three days, raise volume in small steps. If it has not recovered after about five days, move to the next level.

Serious: score between 60 and 80%.

Critical: score below 60%, hard bounces that stay above about 2% after a list cleanup, a Low or Bad reputation in Postmaster Tools, or a blocklisting.

  • Pause all sending on the mailbox, warmup included, for about a week. This short full pause is the one time warmup goes off.
  • Review blocklists and Postmaster Tools.
  • If the domain is several months old and only shows Low, resume warmup only at low volume and increase it over four to eight weeks before any campaign.
  • Retire the domain if reputation is Bad.

When a domain is not worth saving

Some domains are not worth rescuing: a Bad reputation, a blocklisting that is not lifted, or replies at zero across several campaigns. A new domain costs a registration fee plus about three weeks of warmup. A recovery attempt often takes longer and may not work. Retiring a sending domain is normal maintenance, not a failure.

Key takeaways

  • Plan on two to three weeks of warmup only, and wait for a score steadily above about 90% before any cold email.
  • Ramp campaign volume over one to three weeks, depending on the daily cap, and keep warmup on except during a short full pause after a critical drop.
  • Keep spam complaints under 0.1%, as Google recommends, and hard bounces under about 2%, a common convention.
  • Set up Google Postmaster Tools on every sending domain, and remember that a low spam rate or an empty dashboard is not proof of health.
  • React within days: reduce volume first, pause next, and replace the domain when reputation is Bad.