Warming Up IP Addresses

A new IP address is a stranger knocking on the door. Email providers have no idea who you are. You could be a legitimate business sending newsletters to subscribers who actually signed up. Or you could be a spammer who just spun up fresh infrastructure to avoid the blacklists that caught your last operation.

Providers can't tell the difference. So they watch.

IP warm-up is the process of proving you belong—gradually building reputation through consistent, predictable, well-received email until providers trust you enough to deliver your messages reliably.

Why Providers Are Suspicious

New IPs have zero reputation. When you start sending from one, providers have no historical data to judge you by. They don't know if your subscribers want your email or if they'll immediately mark it as spam.

Sudden volume makes it worse. Going from 0 to 50,000 emails in a day looks exactly like a compromised server, a spam operation, or a botnet. Legitimate senders don't usually behave that way.

So providers throttle unknown senders. They intentionally slow down or defer mail from new IPs to observe your behavior over time. This protects users while giving legitimate senders a chance to prove themselves.

Warm-up works because it demonstrates what spammers can't fake: consistent sending, engaged recipients, and clean metrics over weeks. Spammers need to send fast before they get blocked. Legitimate senders can afford to be patient.

The Warm-Up Process

Week 1: Foundation

Start extremely slowly:

Day 1: 50-100 emails
Day 2: 100-200 emails
Day 3: 200-400 emails
Day 4: 400-800 emails
Day 5: 800-1,500 emails
Day 6: 1,500-3,000 emails
Day 7: 3,000-5,000 emails

Send only to your most engaged subscribers—people who opened or clicked recently, who've been active in the last 30 days, who've never complained. These recipients will engage with your email, teaching filters that your mail is wanted.

Week 2: Expansion

Continue doubling every 1-2 days:

Day 8-9: 5,000-10,000 emails
Day 10-11: 10,000-20,000 emails
Day 12-13: 20,000-40,000 emails
Day 14: 40,000-60,000 emails

Expand to users engaged in the last 90 days. Still avoid completely inactive subscribers.

Week 3-4: Scale

Reach your target volume:

Day 15-17: 60,000-100,000 emails
Day 18-21: 100,000-150,000 emails
Day 22-28: 150,000-250,000+ emails

Now you can include your broader audience—all reasonably engaged users, even less active segments.

Week 4+: Full Volume

Achieve and maintain full sending capacity. The warm-up is complete, but monitoring never stops.

Adjusting Based on Signals

The schedule above is a guideline. Reality will tell you whether to speed up or slow down.

Engagement rates tell you how recipients feel:

• Excellent (>30% opens, <0.1% complaints): Double volume daily. Your audience loves you.
• Good (20-30% opens, <0.2% complaints): Follow the standard schedule.
• Moderate (15-20% opens, <0.5% complaints): Slow down. Smaller volume increases.
• Poor (<15% opens, >0.5% complaints): Pause. Investigate. Something's wrong.

Delivery signals tell you how providers feel:

• Everything delivered: Keep increasing.
• Deferrals (4xx errors): Normal during warm-up. Don't increase volume until they decrease—providers are throttling you.
• Hard rejections (5xx errors): Stop. Investigate. Fix the issue before continuing.

Provider-specific problems:

If Gmail is blocking you but Microsoft isn't, slow down specifically for Gmail. Send only to your most engaged Gmail users. Wait for metrics to improve. Each provider evaluates you independently.

Target Volume Guidelines

How quickly you warm up depends on where you're headed:

What Must Be Perfect During Warm-Up

Authentication: SPF, DKIM, and DMARC must pass 100% of the time. Any authentication failure during warm-up significantly damages your emerging reputation. This isn't the time for configuration errors.

List hygiene: Remove hard bounces immediately. Remove soft bounces after 2-3 attempts. Sending to invalid addresses signals carelessness—or worse, a purchased list.

Consistency: Send at the same time of day, on the same days of week, with steadily increasing volume. Don't skip days. Don't send sporadically. Predictability is how you prove you're legitimate.

Content quality: Send your best content during warm-up. Highly targeted. Highly relevant. No spam trigger words, proper HTML formatting. First impressions matter.

Common Mistakes

Don't send to purchased lists. Ever, but especially during warm-up. Purchased lists contain spam traps, invalid addresses, and people who never asked to hear from you. This will destroy your warm-up.

Don't include completely inactive users. If someone hasn't opened an email in two years, they're not going to save your warm-up—they're going to tank it.

Don't skip days. Consistency proves legitimacy. Sporadic sending looks suspicious.

Don't ignore deferrals. When providers throttle you, they're telling you to slow down. Listen.

Don't warm up multiple new IPs simultaneously unless you know what you're doing. Each IP needs its own warm-up attention.

Warming Up Multiple IPs

If you need multiple sending IPs, stagger the warm-ups:

IP 1: Week 1-4
IP 2: Week 2-5 (start in week 2)
IP 3: Week 3-6 (start in week 3)

Or warm them together by splitting volume evenly across all IPs, with each following the same schedule. Monitor each IP separately—some providers see them as one sender, others don't.

Consider dedicating IPs to different purposes. Transactional email (password resets, receipts) warms up faster and gets more leeway. Marketing email faces more scrutiny. Separating them protects your transactional delivery from your marketing reputation.

Provider-Specific Notes

Gmail: The most stringent. Heavily weights user engagement. Use Google Postmaster Tools to monitor your domain and IP reputation. Gmail can be slow to accept high volume—patience and engagement are your tools.

Microsoft (Outlook.com, Office 365): Register for SNDS (Smart Network Data Services) to monitor your reputation. More predictable than Gmail. Watch for green/yellow/red status changes.

Yahoo/AOL: Register for feedback loops to receive complaint notifications. Moderate warm-up requirements. Handle bounces promptly.

Apple Mail: Less transparent than others. Follow general best practices and monitor delivery rates.

Reactivating Dormant IPs

IPs that haven't sent in months lose reputation. How much re-warming they need depends on how long they've been dormant:

Special Situations

Acquired users: When you acquire subscribers from another sender, they've never engaged with your infrastructure. Warm up more conservatively. Send highly relevant content. Watch for engagement drops—these users didn't choose you.

List migration: Moving to a new email service provider means a new sending IP. Your subscribers may not recognize the new infrastructure. Consider a re-permission campaign. Use a conservative warm-up schedule.

Seasonal senders: If you only send heavily during specific periods (retail during holidays), you may need to warm up annually. Consider maintaining some year-round sending to preserve reputation. Start holiday warm-up 6-8 weeks before peak volume.

Monitoring Tools

Provider tools:

• Google Postmaster Tools (Gmail reputation, spam rate, authentication)
• Microsoft SNDS (IP reputation, complaint data)
• Yahoo/AOL Feedback Loops (complaint notifications)

Third-party monitoring:

• MXToolbox (blacklist monitoring)
• Sender Score (reputation scoring)
• Your ESP's built-in analytics (delivery rates, bounces, complaints)

When Things Go Wrong

High deferrals (4xx errors, "deferred due to volume"): Don't increase volume. Wait for deferrals to decrease—usually 24-48 hours. Providers are telling you to slow down.

Hard blocks (5xx errors, blacklist appearances): Stop sending immediately. Investigate the root cause. Clean your lists thoroughly. Request blacklist removal if applicable. Resume carefully only after fixing the issue.

Low engagement (<10% opens, >0.5% complaints): Pause warm-up. Review your content and targeting. Send only to your most engaged users. Don't continue until you've improved relevance.

After Warm-Up

Warm-up is complete, but reputation is ongoing.

Maintain consistent volume. Don't suddenly jump from 100K to 500K emails. If you need to grow beyond your warm-up target, increase 20-30% per week maximum.

Keep monitoring. Daily checks can become weekly, but never stop watching your metrics.

Handle problems immediately. Bounces, complaints, authentication failures—address them the day they appear.