Not even the perfect email copy and a clean, segmented list of leads will save you if your email account is flagged with ESPs. You may have missed one of the key steps for effective outreach: the email warm up process. You can warm up your account manually or by using an email warmup tool.
However, you can also go one step further and use an email warmup API. The email API requires a little bit of technical know-how, but it’s the easiest way to warm up an account and get it ready for full-scale outreach.
Here’s what an email warmup API is and how it helps with email communication. We’ll also show you some of the best warmup APIs to consider for your campaigns.
What is an email warmup API?
An email warmup API is a programmatic way to warm up email inboxes without using a dashboard or manual setup. It lets developers plug email warmup directly into their own systems, such as outreach tools, CRMs, or internal scripts.
Here is what it actually does.
What an email warmup API does
The API connects to one or more mailboxes and gradually builds sender reputation by simulating normal human email behavior. Instead of blasting cold emails, it creates controlled activity that mailbox providers expect from real users.
Typical actions include:
- Sending a small number of emails each day and slowly increasing volume
- Receiving replies to those emails
- Opening emails
- Marking messages as important
- Removing emails from spam folders when needed
All of this happens automatically, based on rules you set through the API and the connected API key.
How it works, step by step
- Mailbox connection
You connect Gmail, Outlook, or custom SMTP inboxes via OAuth or credentials, ideally with full API access in place. - Warmup rules
You define limits such as daily send count, ramp up speed, working days, and pause conditions. - Network interaction
The API sends emails between real or controlled inboxes in a warmup network. These emails look natural, not templated blasts. - Reputation signals
Mailbox providers see replies, opens, and clean inbox placement, which builds trust for the domain and inbox. - Monitoring and control
You track spam placement, sending health, and errors through API responses or webhooks.
Why teams use an API instead of a UI
- You manage hundreds or thousands of inboxes
- You want warmup to start automatically when a mailbox is created
- You need warmup status inside your own app
- You want to pause or adjust warmup based on campaign performance
What it is not
- It does not fix bad copy, poor targeting, or spammy domains
- It does not guarantee inbox placement for reckless sending
- It is not a one time setup, warmup often runs continuously
Simple example use case
A sales platform creates a new inbox for a customer. The system automatically:
- Starts warmup at 5 emails per day
- Increases volume over 14 days
- Pauses warmup when real campaigns begin
- Resumes warmup during low sending periods
How does an email warmup API help with email deliverability?
An email warmup API helps deliverability by controlling how mailbox providers first experience a new inbox or domain. It creates the right signals early, before real outreach starts, and keeps those signals steady over time.
Here is how it helps in concrete terms.
It builds sender reputation gradually
Mailbox providers watch new senders closely. Sudden volume spikes are a red flag. A warmup API enforces slow, predictable growth in sending volume, which looks like normal human behavior to email service providers.
Instead of going from zero to hundreds of emails, the inbox might go:
- Day 1: 3 to 5 emails
- Day 7: 15 to 20 emails
- Day 14: 30 plus emails
That pacing matters more than most people realize.
It generates positive engagement signals
A warmup API creates:
- Replies to sent emails
- Opens and reads
- Messages staying in the inbox
- Spam folder corrections when needed
These actions tell Gmail and Outlook that the sender is wanted, not ignored.
It protects domains and inboxes from early damage
Once an inbox gets flagged early, recovery is slow and painful. A warmup API reduces the chance of:
- First emails landing in spam
- Domains getting a bad reputation
- Inbox level throttling or blocks
This is especially important for brand-new domains or secondary domains used solely for outreach.
It keeps the reputation stable during real sending
Many teams stop warmup once campaigns go live. That is a mistake.
A warm up API lets warm-up run alongside real outreach, filling gaps on low send days and keeping engagement signals consistent. This helps avoid reputation and deliverability score drops during slow weeks or pauses.
It reacts faster than manual warmup
Because it is programmatic, the API can:
- Pause warmup if spam rates rise
- Reduce volume automatically
- Resume once conditions improve
Manual tools rely on people noticing problems. APIs react immediately based on the reactions of your target audience and email providers.
The best email warmup API choices to help with cold outreach in 2026
An email warm up API can do the heavy lifting and help you bypass spam filters quickly and at a low cost. Here are some of the best warmup tools with API access that you can use to protect your sender reputation.
Warmy.io
Warmy.io is an email warmup tool that helps inboxes build reputation before active outreach. It automates sending and engagement so your emails look familiar to mailbox providers over time.
Top features
- Automated warmup activity sends and receives messages on a schedule to grow positive signals for your inbox.
- Engagement simulations opens messages and replies to create natural interaction patterns.
- Inbox placement tracking shows how your emails are landing so you can watch progress.
- Volume control settings lets you adjust how many actions happen each day as warmup continues.
- Multiple inbox support allows you to warm up more than one email address at the same time.
Pricing is not availably publicly.
Mailivery API
Mailivery is a tool that warms up your inbox by sending and interacting with emails in a network of other warmup accounts. It is designed to improve your sender reputation in the lead-up to real outreach.
Top features
- Warmup sending network exchanges messages with other warmup accounts to increase engagement signals.
- Inbox interaction actions opens emails, marks them important, and simulates replies for better reputation.
- Daily activity controls lets you adjust how many warmup interactions occur each day.
- Spam fix routines detects placement issues and corrects them by interacting with misclassified messages.
- Progress monitoring gives you insights on how warmup activity affects your inbox health.
Pricing starts at $29 per month.
Mailreach API
Mailreach is a deliverability and inbox warming solution that focuses on improving placement before and during active sending. It combines automated warmup with analytics on how emails are treated by providers.
Top features
- Automated warmup workflows build out sending and receiving action patterns to strengthen inbox reputation.
- Inbox placement insights tracks whether your email lands in inbox, promotions, or spam.
- Adaptive volume pacing increases daily activity in a way that fits your domain history and usage.
- Multi-account support lets you warm up more than one mailbox with coordinated controls.
- Warmup API support enables integration where warmup status and activity can be managed programmatically.
Pricing starts at 25EUR per inbox per month.
Warmforge
Warmforge is an email warmup platform designed to improve inbox placement before live campaigns start. It focuses on building positive sending signals with automated activity that mimics real user behavior.
Top features
- Automated warmup sequences sends and receives emails each day to strengthen sender reputation.
- Engagement simulation generates opens and replies that mailbox providers count as positive signals.
- Warmup pacing settings lets you set daily send limits and growth speed based on your needs.
- Multi-inbox support warms up several mailboxes at once without extra manual setup.
- Deliverability feedback shows trends in placement so you can adjust warmup if needed.
Pricing starts at $9 per slot per month.
InboxAlly
InboxAlly is a deliverability and warmup platform focused on inbox placement rather than outreach automation. It is used by teams that want tight control over reputation signals before sending real campaigns.
Top features
- Engagement based warmup uses opens, replies, and inbox interactions to build sender reputation.
- Inbox placement monitoring shows whether emails land in inbox, spam, or promotions.
- Custom warmup pacing adjusts volume increases based on inbox health.
- Multiple inbox support warms up many mailboxes at the same time.
- API access enables programmatic warmup management and status checks.
Pricing starts at $149 per month for plans that include warmup features and API access.
TrulyInbox
TrulyInbox is a lightweight email warmup tool aimed at improving inbox placement for new or recovering inboxes. It focuses on continuous background warmup rather than campaign level automation.
Top features
- Continuous warmup cycles keeps inbox activity steady even during low send periods.
- Inbox engagement actions creates replies and positive signals that mailbox providers expect.
- Spam folder correction moves emails out of spam to reinforce inbox placement.
- Multi inbox handling supports warming up several email accounts in parallel.
- Warmup API endpoints lets you manage warmup activity through your own systems.
Pricing starts at $15 per month.
Lemwarm
Lemwarm is an email warmup tool built by the Lemlist team, focused on preparing inboxes for cold outreach. It runs warmup activity automatically and is commonly used alongside outbound platforms where inbox reputation matters from day one.
Top features
- Automated inbox warmup sends and receives emails daily to build trust with mailbox providers.
- Reply and open simulation generates real engagement signals instead of empty send volume.
- Spam recovery actions pull messages out of spam to correct negative placement signals.
- Warmup scheduling controls lets you control daily volume and ramp up speed.
- API based control allows warmup to be started, paused, or monitored programmatically.
Pricing starts at $24 per inbox per month.
Mailgun
Mailgun is an email delivery platform built for developers who send email through APIs at scale. It includes automated dedicated IP warmup, which helps new sending IPs build a reputation before higher volume traffic is introduced.
Top features
- Automated IP warmup gradually increases sending volume on a dedicated IP to establish trust with mailbox providers.
- Email sending API full programmatic control over transactional and bulk email sending.
- Dedicated IP addresses let you own and manage your sender reputation instead of sharing it with other senders.
- Delivery event tracking provides visibility into sends, bounces, complaints, and other delivery signals.
- Suppression lists automatically blocks problem addresses to reduce reputation damage during warmup.
Pricing starts at $15 per month.
Reply.io
Reply.io is a sales engagement platform that combines email outreach with multi-channel sequences. It includes an email warmup API that helps you build sender reputation before or during outbound campaigns so your messages land in inboxes more often.
Top features
• Email warmup API lets you start and monitor warmup programmatically inside your own workflows.
• Reply sequences combine warmup with automated follow ups across email, social, and calls.
• Engagement analytics tracks opens, clicks, replies, and deliverability signals to inform adjustments.
• Mailbox health dashboard shows reputation indicators and issues so you can intervene early.
• CRM integrations connects with popular CRM tools so contact and warmup data syncs with your sales records.
Pricing starts at $49 per user per month.
Get better deliverability (and value for money) with Woodpecker
Warmup APIs solve one part of the deliverability problem, but they stop short once real outreach begins. You still need a separate tool to manage campaigns, sending limits, follow-ups, replies, and ongoing reputation health.
This is where Woodpecker comes in and combines inbox warmup and outbound outreach in one place.
Warmup in Woodpecker runs in the background and adjusts automatically based on how much real email you are sending. When campaigns slow down, warmup keeps engagement signals steady. When campaigns ramp up, warm-up steps back so volumes stay natural. You do not need to pause one tool and start another or guess when to switch modes.
On top of that, Woodpecker gives you controls that pure warmup APIs do not cover. Daily send limits, reply-based follow-ups, bounce protection, domain-level monitoring, and inbox health checks are all in one place. Deliverability decisions are based on real campaign data, and not just simulated activity.
For teams running cold outreach at scale, Woodpecker gives warmup, outreach, and deliverability protection working together instead of stitched across multiple tools.