What is email warm-up?
Email warm-up builds a new mailbox's sending reputation before outreach. Warm-up networks and gradual volume ramp-up compared, and when you need each.
By the PromptLab team27 October 20264 min read
Definition
Email warm-up
Email warm-up is the process of building a new mailbox's reputation with mail providers before it sends outreach at volume. It is done either through a warm-up network that exchanges emails with other mailboxes, or by raising the number of real emails sent a little each week.
Gmail and Outlook watch how a mailbox behaves. A mailbox that sent nothing yesterday and 300 emails to strangers today looks like a spammer, and its mail goes to junk. Warm-up gives the mailbox a history first, so its sales emails reach the inbox.
Why new mailboxes need warm-up
A new mailbox, or a mailbox on a new domain, has no track record. Mail providers can't tell yet whether its owner is a rep writing to distributors or a bot sending junk. So they are cautious, and a sudden burst of email tips them towards spam.
This matters most in three situations:
- A new sending domain. Many teams send cold email from a separate domain to protect the main one, and that domain starts with no history.
- A new rep. Their mailbox is fresh, and their first outreach campaign is often their biggest send.
- A mailbox that has been quiet. A shared sales@ address that was barely used is in the same position.
Warm-up networks vs gradual ramp-up
There are two ways to warm up, and they work differently.
| Warm-up network | Gradual volume ramp-up | |
|---|---|---|
| How it works | Your mailbox trades automated emails with a pool of other mailboxes, which open them, reply and move them out of spam | Your mailbox sends real emails to real buyers, starting low and rising each week |
| Who it emails | Other users' mailboxes in the pool | Your actual prospects and customers |
| What it needs | A warm-up tool connected to your mailbox | A sending tool with daily limits |
| Strength | Builds activity before outreach starts | Every email sent is a real one |
| Watch out for | Your mailbox's history is partly made of automated traffic | Volume grows slowly, so big campaigns take time |
Many teams do both: a warm-up network for a new domain, plus a slow start on real sends. Either way, warm-up is not a cure. If the domain lacks SPF, DKIM and DMARC, or the list is full of dead addresses, no warm-up will fix it. Our cold email deliverability checklist covers the rest.
An example from product sales
This example is made up.
A packaging maker hires a new rep to open accounts with food producers. The rep gets a mailbox on a new sending domain. In week one the mailbox sends 10 outreach emails a day, plus normal emails to colleagues and existing customers. In week two it sends 20 a day, and in week three 30. By the time the rep's main campaign to 400 food producers starts, the mailbox has three weeks of steady, real sending behind it.
Which tools offer warm-up networks
Dedicated cold email tools include warm-up networks. Prices below were checked on each vendor's pricing page on 20 October 2026:
- Instantly says you can connect and warm up as many mailboxes as you need on every plan, with advanced warm-up from its Scale plan. Pricing starts at $76.50 a month per workspace, billed annually.
- lemlist includes its lemwarm warm-up tool on all plans, from $55 a month billed annually.
- Smartlead includes a warm-up pool whose tier rises by plan, from $32.50 a month billed annually.
PromptLab takes the second route. It has no warm-up network. Instead, it ramps up the daily volume of new mailboxes over three weeks. See Instantly vs PromptLab.
Related terms
- An email sequence is what usually goes out once a mailbox is warmed up.
- Sender reputation is the score mail providers keep for your domain and mailbox; warm-up builds it.
- SPF, DKIM and DMARC are the DNS records that prove your email is genuine.
How PromptLab handles it
PromptLab's campaigns send from your own Gmail, Outlook or IMAP mailboxes. Daily sending limits per mailbox type are built in, and a new mailbox ramps up over three weeks: 10 emails a day in week one, 20 in week two and 30 in week three. You can turn the ramp-up off for mailboxes that already have a history. Campaigns rotate up to 20 mailboxes and leave a random gap between emails, so volume is spread out.
Frequently asked questions
How long does email warm-up take?
A few weeks is common for a new mailbox. PromptLab's built-in ramp-up runs over three weeks, rising from 10 to 30 emails a day. A new domain may need longer before it sends at full volume.
Is email warm-up necessary?
For a new mailbox or a new sending domain, some form of it is. A mailbox that jumps straight to high volume is likely to land in spam. For an established mailbox with a long history of normal email, it matters less.
Does PromptLab have email warm-up?
PromptLab ramps up the daily volume of new mailboxes over three weeks, but it has no warm-up network. Instantly, lemlist and Smartlead include warm-up networks.
Can warm-up fix bad deliverability?
Not on its own. Missing SPF, DKIM or DMARC, a list with many dead addresses or heavy, link-filled emails will still land in spam. Fix those first, then warm up.