Email Deliverability and IP Warm-Up

Email deliverability directly impacts the success of your campaigns. While inbox placement is influenced by multiple factors—many outside a sender’s control—there are several practices brands can follow to improve outcomes. This chapter outlines the key elements that affect deliverability and provides practical steps for IP warm-up to help ensure your messages reach the intended inboxes and optimize email engagement.

What impacts your email deliverability and IP Warm Up basics

Inbox placement depends on both who is sending the email and the email’s content. Internet Service Providers (ISPs) and inbox providers assess sender reputation, email infrastructure, and engagement patterns before accepting emails.

The following factors typically influence deliverability:

• Missing or misconfigured SPF/DKIM authentication
• Poor IP reputation
• Spam-triggering keywords in subject lines or body
• Suboptimal email infrastructure health
• Negative sender behavior history
• New or low-age sending domains
• High bounce or complaint rates
• Receiving server settings and throttling
• Non-compliance with CAN-SPAM and opt-in/opt-out regulations

Why New Senders and IPs are Treated with Caution

Inbox Service Providers (ISPs) are cautious of unknown or new senders—especially when large volumes of emails come from unrecognized IP addresses. This caution stems from the widespread abuse of email marketing by spammers.

Without proper IP warm-up, ISPs may block these emails or route them to spam folders. Deliverability improves when early emails receive meaningful engagement, such as opens, clicks, or replies. These interactions help build your IP reputation and establish you as a trustworthy sender.

To avoid delivery issues, it’s essential to start with low volumes and gradually scale to your expected daily traffic, while continuously monitoring and optimizing engagement.

How many IPs do you need?

The number of IPs needed directly depends on your daily email volume and the types of recipient domains you target. Larger Inbox Providers (ISPs) can typically handle high traffic from a single IP, while smaller ones may require you to distribute the load across multiple IPs.

Most ISPs enforce sending limits per IP—either hourly or daily. Your IP reputation also affects throughput: higher-reputation IPs can handle more volume, while lower-reputation ones may require more IPs to deliver the same volume.

As a general guideline, one IP can support up to 1 million emails per day for marketing emails. For transactional emails, especially when spread throughout the day, this capacity may go up to 2 million emails per day per IP.

The IP Warm Up Process

Warming up IPs is crucial to build a strong sender reputation and ensure high email deliverability. Follow these steps to effectively warm up your IP addresses:

Step 1: Understand Reputation Stability

• IP reputation data is typically stored for 30 days by ISPs.
• Avoid sudden spikes or major changes in email volume, content type, or recipient domains within this period.
• Gradually increase email volume to maintain a stable reputation.

Step 2: Start with Internal Testing

• Send initial emails to trusted colleagues and internal contacts.
• Encourage recipients to open, read, click links, and mark emails as safe if found in Spam.
• Replies from recipients also help boost trust signals.

Housekeeping Tip: Ensure SPF and DKIM records are correctly set up and use valid, monitored sender and reply-to addresses.

Step 3: Target Your Most Engaged Users

• Analyze recent subscriber behavior (past 2–3 months) and select users who consistently engage with your emails.
• If no engagement data is available, start with your most recent sign-ups.
• Prioritize sending transactional emails (e.g., confirmations, welcome emails) as they tend to generate higher engagement than promotional content.

Step 4: Gradually Increase Email Volume

• Follow a staged volume increase such as: 50 → 500 → 5,000 → 50,000 → 500,000 → 5,000,000
• Avoid large volume jumps to reduce the risk of being flagged as spam.

Step 5: Distribute Traffic Across ISPs

• Evenly spread email sending across major inbox providers (Gmail, Yahoo, Outlook, etc.) daily.
• Balanced ISP coverage helps build consistent sender reputation and avoids throttling.

Step 6: Monitor and Adjust

• Track deliverability and engagement metrics regularly.
• If performance is strong, consider accelerating the warm-up pace.
• If issues arise, slow down the process and focus on high-engagement recipients.

Typical Timeline

• A steady warm-up process usually takes 45–60 days.
• This timeframe allows scaling up to around 1 million emails per day per IP.

Fall-Back Plan for Underperforming IPs

Sometimes, IP reputation growth doesn’t go as planned. If you notice increased email blocks or low engagement, slow down the IP warm-up process. Focus on sending high-quality emails to your most engaged recipients. This allows your reputation to recover before scaling back up to your desired volume.

Remember, low engagement doesn’t always mean poor gateway performance. Since IP reputation is only one factor, review your email content to ensure it matches your target audience’s preferences and expectations.

Recommendations

• Align the number of IPs with your daily email volume.
• Prioritize building and maintaining a strong IP reputation for better deliverability.
• Start IP warm-up using internal emails, then progress to your most engaged recipients.
• Focus on driving engagement during the warm-up phase.
• Increase sending volume gradually, monitoring deliverability and engagement.
• Distribute email traffic evenly across major recipient ISPs.
• Choose campaign types that encourage recipient interaction and avoid spam triggers.
• Spread email sends throughout the day instead of sending large blasts.
• Regularly monitor campaign performance to identify and address issues quickly.
• Ease into any changes in sending profile to preserve your IP warm-up progress.