How inbox warm-up works, and how long it takes

A new inbox on a new domain can send email on its first day, but the mailbox providers receiving that email know nothing about it yet. Warm-up is the period where you build that history on purpose: small, steady sending that gets opened and answered, rising slowly, before you ask the inbox to carry real campaign volume.

This guide covers what warm-up does, how long it commonly takes, how to tell whether it is working, and what to do once it is done.

What warm-up is

Gmail, Outlook and the other large providers decide where each message lands using what they have learned about the sender. How much does this sender usually send? Do people open, reply to, or delete its mail? Does anyone mark it as spam? Do its messages bounce?

A new inbox has no answers to any of those questions. Warm-up gives it some: gradual sending plus positive engagement, so there is a track record in place before your campaigns start.

Most warm-up tools work in a similar way. Your inbox exchanges messages with a pool of other inboxes taking part in the same program. Those inboxes open the messages, reply to some of them, and move any that land in spam back to the inbox. Each day the volume rises a little. Over time the provider sees a mailbox with a steady, growing history of mail that gets opened and answered.

Why new domains and inboxes start with no reputation

Reputation attaches to the things a provider can observe over time. For a Google Workspace inbox, the most important one is your sending domain: the domain in your From address. Google Workspace mail leaves from Google's own servers, so you do not build or control IP reputation yourself. The domain's record is the part that is yours.

A domain registered last week has no record at all: no volume history, no engagement, no complaints. Providers treat that as unknown, not as good, and some filters are extra cautious with very young domains. A jump from nothing to full campaign volume is the pattern spammers produce with fresh domains, so it tends to be filtered like one.

Google says as much in its email sender guidelines: start with a low sending volume to engaged users, increase it slowly over time, and avoid sudden volume spikes if you do not have a history of sending large volumes. It also asks senders to send at a consistent rate rather than in bursts.

What a sensible ramp looks like

Mailbox providers do not publish a warm-up schedule, so exact daily numbers quoted as a rule are guesses. What matters is the shape: start low, rise gradually, keep it steady, and only add campaign mail once the warm-up mail is landing well.

The table below shows that shape. It is an illustration, not a rule: your warm-up tool's defaults and your own results should set the actual pace.

A few habits hold whatever tool you use:

• Authenticate first. Set up SPF, DKIM and DMARC for the domain before the first message. Warming a domain that fails authentication builds the wrong history.
• Keep the daily volume steady. Skipped days followed by a burst undo much of the point.
• Raise volume in small steps. Hold, or step back, when results slip.
• Make the inbox look like a person's. A real name and a plain signature fit the pattern of a working mailbox.

Signs warm-up is working, or not

Your warm-up tool usually reports how many of its messages reached the inbox and how many landed in spam. Read over several days, that is the main signal. Check it alongside the others:

If you send enough mail to personal Gmail accounts, Google's Postmaster Tools can also show your domain's spam rate. Google asks senders to keep the spam rate reported there below 0.3%, and recommends keeping it below 0.1% (Google's sender guidelines).

When the signals go the wrong way, slow down before you push on. Check authentication first, then look for what changed: a burst of volume, a list with bad addresses, a new piece of copy. Warm-up cannot repair a domain that is being used to send mail people do not want.

How long warm-up takes

Commonly a few weeks. There is no fixed number, because the time depends on:

• How much you plan to send. A light daily load per inbox needs less history than a heavy one.
• The age and history of the domain. A brand-new domain starts from nothing.
• Consistency. Days off and bursts slow it down.
• Authentication. A domain that fails SPF, DKIM or DMARC will not build a good record however long you wait.

Treat "done" as a result rather than a date. The warm-up inbox rate has been steady for a while, nothing bounces, authentication passes, and a small first batch of campaign mail behaves the same way.

Pre-warmed inboxes as an option

If you would rather not wait out the ramp yourself, one option is to buy inboxes that have already been through it. InboxesNow sells pre-warmed Google Workspace inboxes, and every inbox is warmed for at least 21 days before it goes on sale.

Pre-warmed does not skip the steps above: the ramp still happened, just before you bought. It does not replace good sending habits afterwards either. For the trade-offs either way, see pre-warmed inboxes versus warming your own.

What to do after warm-up

Warm-up gets an inbox to the starting line. What you do next decides whether the reputation holds.

1. Keep volume per inbox moderate. Add inboxes and domains rather than pushing each one harder. InboxesNow plans around 30 campaign emails per inbox per day, alongside warm-up, which is 90 per domain. To size that for your own volume, see how many inboxes and domains you need.
2. Keep warm-up running if your tool supports it. Ongoing warm-up keeps a base of positive engagement under your campaign mail.
3. Ramp campaigns too. Start campaign volume small and step it up, the same way warm-up did.
4. Verify your lists. Bounces from invalid addresses are one of the quickest ways to undo a good warm-up.
5. Watch trends weekly. Replies, bounces, complaints and placement over a week tell you more than any single day.

If you buy from InboxesNow to use with a sequencer other than Instantly, you release the domain to get each inbox's address and password, and InboxesNow's warm-up of those inboxes stops the moment you release it. If you want warm-up to continue, turn it on in your sequencer. The steps are in using pre-warmed inboxes with any sequencer.

How a warm-up record makes this visible

Warm-up usually happens out of sight. When someone else has done it, you are told an inbox is warmed and asked to take that on trust.

A warm-up record shows the history instead. On InboxesNow, each domain shows its warm-up record before you buy:

• days warmed
• a daily warm-up chart: sent versus reached the inbox
• the 7-day warm-up inbox rate
• a warm-up health score, one per domain

With that in front of you, you can run the same checks described above. Has the domain been warming long enough? Is the daily line steady? Are warm-up messages reaching the inbox? If you would rather not pick domains by hand, "Auto-pick best" chooses them by 7-day warm-up inbox rate.

Next to the warm-up record, each domain and each inbox also shows an inbox placement record: the results of daily tests to seed inboxes at Google Workspace, Outlook and Gmail, with the inbox rate per provider over the last 7 days. The warm-up record shows how the inbox has been sending; the placement record shows where test messages landed. Neither is a guarantee of where your own campaigns will land.