5 Best Email Warmup API Providers for Better Deliverability
When you search for an email warm-up API, you’ll see a lot of tools claiming they can fix deliverability.
But not all of them work the same way - some just fake inbox activity, some limit how many mailboxes you can use, and only a few give you real APIs that scale automatically.
We’ve narrowed it down to the APIs that actually deliver:
- Connect directly with Gmail, Outlook, Microsoft 365, or SMTP.
- Show real inbox vs spam placement, not just basic checks.
- Support scaling from a single sender to thousands of mailboxes.
This guide breaks down what makes each provider unique and how to pick the right one for your workflow.
Key Takeaways: 5 Best Email Warmup API Providers
- Best for Deliverability Control & Monitoring – Warmforge
- Best for B2B Cold Outreach & Multi-Account Management – Mailreach
- Best for Easy Email Warm-Up Integration into Existing Platforms – Warmy.io
- Best for Scalable Transactional Email Delivery – Mailgun
- Best for Scalable Warm-Up and Team Integrations – Mailivery
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What is an Email Warmup API and How Does It Work?
An Email Warmup API connects directly to your inbox and improves deliverability by making your email account look trusted.
Instead of logging in and sending test emails manually, the API automates the entire email warmup process in the background, builds a positive sender reputation, and ensures your emails reach the inbox.
How Does an Email Warmup API Work?
An Email Warmup API improves deliverability by simulating natural email activity on your account. Here’s how it works step by step:
- Secure connection to your inbox
The warm-up API connects to your mailbox (Google Workspace, Outlook, or other providers) using OAuth, IMAP, or SMTP credentials. This gives it programmatic control to send and receive test emails.
- Send small batches of emails
At first, the API sends just a handful of emails per day. This prevents sudden spikes in sending activity that might trigger spam filters.
- Gradually increase sending volume
Over days and weeks, the system ramps up the daily email count, mimicking the natural growth of a real user or business account.
- Generate positive signals
Some warm-up tools automatically open emails, mark them as important, or even reply within their private warm-up networks. The idea is to create “positive engagement signals” that help improve sender reputation.
- Build sender reputation
As the account maintains consistent sending patterns, low bounce rates, and authenticated DNS (SPF, DKIM, DMARC), mailbox providers start treating the domain as trustworthy, which improves inbox placement for real campaigns.
API trains your inbox in the background so your cold emails, newsletters, or campaigns have a much better chance of landing in the inbox instead of spam.
What to Look for in an Email Warm-Up API
If you’re comparing email warm-up APIs, don’t just look at who promises “better deliverability.” What really matters is how easily the API fits into your system and keeps things running smoothly.
Here’s what to check:
- Secure connections
Make sure the API uses safe login methods like OAuth or API keys, and protects your inboxes with IP allowlisting. - Simple setup and control
You should be able to add inboxes, start or stop warm-up, and see results — all through simple API calls. - Custom settings
Look for options to set daily send limits, control ramp-up speed, and adjust for Gmail, Outlook, or custom SMTP. - Real-time data
The API should show live stats like inbox placement, spam rate, bounces, and reputation - not just “success” checkmarks. - Smart webhooks
You should get automatic alerts when something changes, like a domain issue or sudden deliverability drop. - Built-in protection
A good API slows down or pauses sending when it detects risk, so your domains stay safe. - Easy scaling
If you manage many inboxes, the API should handle bulk actions without breaking. - Clear pricing
You shouldn’t have to guess how much you’ll pay. Look for simple, per-mailbox or per-usage pricing.
In short, pick an API that makes things easy, gives you control, and keeps your inboxes healthy automatically.
How We Picked the 5 Best Email Warmup API Providers
To create this list, we focused on the factors that actually improve deliverability and make an Email Warmup API practical for daily use.
Each provider was reviewed and checked against the following points:
- Whether the API can be set up quickly so inboxes start warming without technical roadblocks.
- How strong the automation is, including reply simulations, spam rescue, and natural engagement.
- If inbox placement improved during testing, with fewer emails landing in spam and a stronger sender reputation.
- Whether it connects smoothly with Gmail, Outlook, custom SMTP, and CRMs without complex workarounds.
- Whether the reporting is clear enough for anyone on the team to track results without confusion.
- How long does it take for a new domain to move from zero to being ready for live campaigns?
This way, the providers we selected are not just feature-packed but also proven to deliver real improvements in inbox placement.
Comparison Table: Best Email Warmup API Providers
1. Warmforge – Best Email Warmup API for Deliverability Control & Monitoring
Warmforge is a complete email warm-up and deliverability platform with its own API that lets you automate almost everything - from warming up mailboxes to checking DNS records and inbox placement.
You can plug it into your own systems or dashboard and keep your sender reputation healthy without manual work.
Warmforge API Features
The Warmforge API lets you:
- Warm up Gmail, Outlook, Microsoft 365, and custom domains.
- Run placement tests to see if your emails land in the inbox, promotions, or spam.
- Check DNS and MX records to make sure your setup is correct.
- Monitor blacklist status and sender reputation.
- Automate all of this inside your own system with simple API calls.
Warmforge API Pros & Cons
Pros:
- Single API covers warmup, placement testing, and deliverability monitoring.
- Connects easily with Google Workspace and Microsoft 365 mailboxes.
- Let's you manage multiple domains and inboxes programmatically.
- Provides Heat Score and reputation tracking through API endpoints.
- Supports controlled ramp-up schedules via API calls.
Cons:
- Needs DNS/SPF/DKIM records set up before the API works properly.
- Historical performance data is limited through the API.
2. MailReach – B2B Cold Outreach & Multi-Account Management API
MailReach is an email warm-up and deliverability platform that improves your sender reputation and helps your emails land in the inbox instead of spam. The Mailreach API lets you bring all these features - warm-up, reputation tracking, and performance monitoring - directly into your own product or system.
MailReach API Features
MailReach provides developer-friendly API access so teams can automate warmup, run spam tests, and monitor deliverability without manual work. The API lets you:
- Warm up Gmail, Outlook, Microsoft 365, and other SMTP accounts.
- Adjust warmup pace and daily volume (up to 100 emails per day).
- Connect to a network of 30,000+ high-reputation inboxes for natural interactions.
- Get real-time performance monitoring and reputation tracking per provider.
- Automate spam testing to check placement and avoid red flags.
Mailreach API Pros and Cons
Pros:
- Well-documented API that’s quick to integrate.
- Let's you adjust warm-up speed, volume, and timing through code.
- Real-time API responses show reputation and performance updates.
- Webhook and Slack support make monitoring fully automated.
- Uses a large live inbox network, so API actions create real, human-like engagement data.
Cons:
- API doesn’t include placement testing or DNS monitoring endpoints.
- No bulk domain or multi-account management via API.
- API pricing and usage limits aren’t clearly shared.
3. Warmy.io – Best for Easy Email Warm-Up Integration into Existing Platforms
Warmy.io API is an integration-ready email deliverability and warm-up solution that lets developers embed Warmy’s tools directly into their own platforms or websites.
Through the API, you can help users automatically warm up mailboxes, improve domain reputation, and reduce spam issues - all without building your own infrastructure from scratch.
Warmy API Features
With the API, you can:
- API for automatic email warm-up and deliverability boost
- Ready-made interfaces for faster setup and lower dev cost
- Simple onboarding with full API documentation and support
- Works with almost any web or desktop application
Warmy.io API Pros and Cons
Pros
- Quick to integrate using existing API endpoints and docs
- Saves development time with plug-and-play functionality
- Improves email reputation and inbox placement via API calls
- Helps teams scale deliverability tools inside their own products
- Offers direct support during API onboarding
Cons
- API access requires a scheduled call (no instant key)
- No public info on API rate limits or usage caps
- Missing advanced endpoints like webhooks or analytics export
- Doesn’t mention placement testing or DNS monitoring APIs
4. Mailgun – Best for Scalable Transactional Email Delivery
Mailgun is a transactional email delivery service and API for developers that lets you send, receive, validate, and optimize emails directly from your own application
It’s designed for teams that need to deliver high volumes of emails quickly and reliably, while keeping full control over tracking, testing, and deliverability through API endpoints.
Mailgun API Features
Mailgun is so its API allows you to:
- Email API for sending, receiving, and parsing transactional messages
- Real-time validation API to remove invalid or risky addresses
- Optimization tools to improve inbox placement and engagement
- Preview and QA API for testing accessibility and formatting
- Scalable infrastructure built for thousands to millions of sends
Mailgun API Pros and Cons
Pros
- Developer-first API with detailed documentation and SDKs
- Combines sending, validation, and optimization in one platform
- Proven deliverability with tools to track bounces and engagement
- Scales easily for transactional and marketing use cases
- The free plan allows testing up to 100 emails per day
Cons
- No built-in mailbox warm-up or reputation management API
- Requires DNS and API setup knowledge for configuration
- Some optimization features come with higher-tier plans
- Less suitable for teams seeking no-code or plug-and-play tools
5. Mailivery – Best for Unlimited Mailbox Warmup via API
Mailivery is an email warm-up and deliverability tool that helps your emails land in the inbox instead of spam.
The Mailivery API extends these capabilities to developers, letting you integrate warm-up, reputation tracking, and team management directly into your own platform or SaaS product.
Mailvery API Features
Mailivery’s API lets you:
- Full API access for warm-up and deliverability automation
- AI-powered interactions with 50K+ active mailboxes
- Customizable warm-up speed, volume, and timing
- Real-time spam and reputation analytics via API
- Blacklist monitoring for 70+ lists
- Embeddable onboarding form and team management endpoints
Mailivery API Pros and Cons
Pros
- One of the few APIs offering AI-based warm-up automation and blacklist monitoring together
- Let's you manage multiple mailboxes and users programmatically - ideal for agencies
- Provides real-time deliverability insights and performance data through API
- Offers custom warm-up controls like pacing and timing for each mailbox
- Includes embeddable onboarding forms, useful for SaaS integrations
Cons
- API is not openly documented - access requires a custom plan
- Lacks placement testing endpoints offered by tools like Warmforge
- Doesn’t expose email validation or DNS monitoring APIs
- Fewer public developer resources compared to Mailreach’s API
- Integration setup may take longer due to limited public documentation
How to Add an Email Warmup API to Your Workflow
Using an Email Warmup API is straightforward once you set it up step by step.
Step 1: Get Your API Key or OAuth Token
Start by getting API access from your warmup provider.
You’ll need this to connect and control inbox warmups.
- Generate an API key or set up OAuth.
- Store keys in a secure vault (not in code).
- Rotate or revoke old keys every 60–90 days.
Step 2: Connect Your Inboxes
You can’t warm what’s not connected.
Add every sending account you’ll use:
- Gmail, Outlook, or Microsoft 365 via OAuth
- Custom domains using IMAP + SMTP
- Tag each inbox by client, team, or campaign
Tip: Make sure ports 587 (SMTP) and 993 (IMAP) are open on custom servers.
Step 3: Verify DNS Setup
Warmup won’t work if your domain isn’t trusted.
Before adding mailboxes:
- Verify SPF, DKIM, DMARC, and MX records.
- Use tools like MXToolbox or Warmforge DNS Check.
- Avoid domains registered less than 14 days ago.
Step 4: Add Mailboxes Using the API
Create and organize mailboxes programmatically.
- Use endpoints such as /mailboxes/create.
- Include metadata like workspace_id or client_id.
- Enable warmup only after DNS passes verification.
Step 5: Start Warmup Gradually
Don’t rush sending. Warmup is about slow trust-building.
- Begin with 2–5 emails per day.
- Increase by 10–15 % every 2–3 days.
- Keep reply ratios around 40–60 %.
- Run for 2–3 weeks before cold outreach.
Step 6: Make Warmups Look Human
ISPs detect fake activity. Keep it natural.
- Send during 8 AM – 6 PM local time.
- Vary subjects, body lengths, and recipients.
- Enable real-looking opens, replies, and spam-to-inbox moves.
⚠️ Avoid fake click signals, they can trigger filters.
Step 7: Monitor Key Metrics
Use your provider’s webhooks or dashboard to track:
- Deliverability score per domain
- Spam and bounce rates
- Reply ratio and inbox placement
- Blacklist and reputation alerts
Set up /events/warmup_status or equivalent webhook to get live updates.
Step 8: Add Safety Nets
Automation should stop problems before they spread.
- Auto-pause warmup if bounces > 3 % or spam > 0.3 %.
- Retry soft bounces after 2–6 hours.
- Disable inboxes with > 5 hard bounces in 24 hours.
Step 9: Define When You’re “Ready”
Know the metrics that mean “go-live”:
- Spam rate < 2 % for 5 days.
- Bounce rate < 1 %.
- Inbox placement > 90 %.
- All DNS and tracking domains are verified.
Step 10: Switch to Maintenance Mode
Once your sender reputation is strong, scale down warmup.
- Send 3–5 warmup emails per day per inbox.
- Keep warmup running during outreach pauses.
- Rotate inboxes to spread sending load.
Tip: If you don’t want to deal with complicated setups, Warmforge’s API is a good starting point because it handles warmup, placement tests, DNS checks, and blacklist monitoring in one place.
What is the Right Email Warmup API Tool?
The right Email Warmup API for you really depends on what you need it for.
- Warmforge, if you want everything in one API - warmup, placement tests, DNS checks, and blacklist monitoring.
- MailReach, if you’re an agency or sales team, spam testing alongside warmup.
- Warmy.io, if AI-driven automation and inbox insights are most important.
- Mailgun, if you need an enterprise-grade email warm-up API that’s built into a complete delivery platform.
- Mailivery, if you want unlimited mailboxes and flexible scaling for SaaS or agencies.
👉 Try Warmforge API - automate warm-ups and boost deliverability fast.