Mailbox warmup
Mailbox warmup is the practice of gradually increasing a new email account's sending volume while generating positive engagement (opens, replies, moving mail out of spam) so mailbox providers build trust in the sender before real campaigns ramp up.
How it works
An automated network exchanges low-volume, high-engagement messages from your mailbox for ~2–4 weeks, then volume scales up as reputation builds.
Why it matters
Sending cold volume from a brand-new mailbox is the fastest way to land in spam. Warmup establishes the engagement history providers look for.
What does a warmup ramp look like day by day?
A conservative hypothetical plan starts a new mailbox at 5 sends on day one and adds about 2 a day. By day 10 it sends roughly 23, by day 16 about 35, and it stops rising at a ceiling of around 40. Real cold sends enter gradually after that, replacing warmup volume rather than stacking on top, so the mailbox's total daily count never jumps.
The exact numbers matter less than the shape: a slow slope, no gaps of several days, and a hard ceiling. If bounces or spam placement appear during the ramp, hold the count flat for a few days before climbing again. The email warmup planner turns a target volume into a dated schedule you can follow.
What can mailbox warmup not fix?
Warmup builds history on a new account. It cannot repair a list full of invalid addresses, a missing DKIM record or copy that recipients report as spam. Once real campaigns start, providers weigh how real recipients react, and a warmed mailbox that sends to an unverified list will lose its standing within days.
It is also no substitute for audience fit. A mailbox that sends well-targeted, verified emails at modest volume often keeps good placement with little warmup, while one that blasts a broad list does badly after weeks of it. Our piece on the mailbox warmup myth covers where the practice helps and where it is oversold.
Should warmup keep running after campaigns start?
Usually a low background level helps, especially for mailboxes whose cold volume swings from week to week. A mailbox that goes quiet for a month and then resumes at full volume looks like a new sender again, so restart the ramp after long pauses. Adding mailboxes on a fresh domain also means starting from the bottom, even if older domains in the program are healthy.
To judge whether a mailbox is ready, look at outcomes rather than the calendar. Send a few test messages to seed accounts at Gmail and Outlook and check they reach the inbox, confirm hard bounces on the first real batch stay near zero, and watch for any spam complaints. If all three look clean, start real sends at a fraction of the ceiling and let them take over from warmup traffic over the following week.
How is mailbox warmup different from IP or domain warmup?
A Google Workspace or Microsoft 365 mailbox sends from the provider's shared IPs, so the reputation you build sits on the account and on the domain, not on an IP you own. IP warmup applies when you control a dedicated sending IP. Domain reputation accumulates across every mailbox on the domain, which is why one badly run mailbox can drag down its siblings.
Autocloz includes a 21-day warmup ramp with adaptive daily ceilings, so each connected mailbox climbs only as fast as its signals allow. Warmup is included on every plan, and the ramp applies separately to each mailbox, so adding a new account to an established domain does not let it skip the climb.
How Autocloz handles it
Autocloz includes a 21-day warmup ramp and adaptive daily ceilings, so mailboxes (and other channels) scale only as fast as their reputation allows.
Free tools for this
No signup required — they run in your browser.
FAQ
How long should mailbox warmup take?
Plan for about three weeks before sending meaningful cold volume, ramping daily sends gradually rather than spiking.
Related terms
Sender reputation is the trust score mailbox providers assign to your sending domain and IP based on how recipients react to your mail — opens, replies, spam complaints, bounces and spam-trap hits. A high reputation lands you in the inbox; a low one routes you to spam or blocks you.
Inbox placement rate (IPR) is the percentage of delivered emails that actually land in the primary inbox rather than the spam/junk folder or a tab like Promotions. Unlike 'delivered' (which only means accepted by the server), IPR measures whether a human is likely to see the message.
Domain reputation is the trust score mailbox providers assign to your sending domain based on its authentication, sending history, engagement and complaint signals. Unlike IP reputation, it follows the domain wherever it sends, which is why it has become the dominant reputation signal as senders share pooled IPs.
IP warmup is the process of gradually increasing sending volume from a new or dormant dedicated IP address while maintaining high engagement, so mailbox providers build trust in that IP before it sends full volume. Sending large volume from a cold IP triggers throttling or spam-foldering because providers have no history to trust.
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