What are the best strategies for warming email sending IPs by individual ISP?

Key findings

• ISP variations: Different ISPs have varying rate limits and filtering algorithms, meaning a one-size-fits-all warming plan may not be optimal. Some are more sensitive to volume spikes than others.
• Top ISPs focus: It is a common and effective practice to focus on warming strategies for the top 5 to 10 major ISPs that account for the largest portion of your audience, such as Google, Microsoft (Outlook/Hotmail), and Yahoo (Oath).
• Gradual volume increase: The fundamental principle of IP warming involves gradually increasing email volume over time. This approach allows ISPs to establish trust with your IP address.
• Reputation building: IP warming is essential for building a positive sender reputation with ISPs. Without it, your emails are more likely to be flagged as spam or blocked outright.

Key considerations

• Audience segmentation: When warming, send to your most engaged subscribers first. This helps demonstrate positive engagement to ISPs. For more on improving deliverability, consider managing inactive email subscribers.
• Monitoring bounces: Closely monitor bounce rates, especially during the initial warming phase. High bounce rates can signal issues with your warming strategy or list quality.
• Consistency: Maintain consistent sending volumes once warmed up. Erratic sending patterns can negatively impact your sender reputation. Understanding what factors influence email deliverability is crucial.
• ISP communication: Some ISPs, like Microsoft, may allow you to pre-register new IPs, potentially accelerating the warming process by applying existing reputation. For more details on IP warming best practices, consider reviewing resources like Twilio SendGrid's guide.

Key opinions

• Individual ISP warming: Many marketers prefer to warm IPs individually for major ISPs like Google, Microsoft, and Yahoo due to their specific rate limits and filtering tendencies.
• Top 5 focus: It is common practice to identify the top 5 ISPs (based on audience reach) and apply specific sending caps to each during the warming period.
• Automation for throttling: Implementing automation to monitor logs for connection limits and throttle sending volume based on those limits is a frequently mentioned strategy.
• Bounce monitoring: Automated systems that detect specific bounce types indicating problems and temporarily suspend sending from affected IPs are highly valued.
• Daily/hourly limits: Setting a maximum percentage of daily volume (e.g., 10%) that can be sent within any given hour helps distribute traffic and prevent spikes.

Key considerations

• Proactive ISP contact: Reaching out to ISPs like Microsoft beforehand can allow them to apply existing reputation to new IPs, potentially shortening the warming cycle.
• Consistency for progression: A rule of thumb is to only increase throttle limits when a significant percentage (e.g., 75%) of the daily allowed volume has been sent consistently for a few days.
• Watch for blocklists: Despite careful warming, issues with blocklists (or blacklists) can still occur. It's crucial to understand what happens when your IP gets blocklisted and how to resolve them.
• Volume milestones: Successful warming allows for significant email volume increases, sometimes reaching tens of millions of emails, but it requires diligent monitoring. For common deliverability issues, review common deliverability issues during new IP warm-up.

Key opinions

• ISP-specific warming: Experts recommend individual warming for major ISPs known for strict rate limits, such as Google, Microsoft, and Yahoo, as well as smaller, more sensitive providers like Comcast or Orange.
• Automated throttling: It is critical to implement automation that checks logs for connection limits and dynamically throttles sending to avoid hitting caps and triggering negative reputation signals.
• Bounce feedback integration: Automation should also detect specific bounce codes that indicate problems and temporarily suspend email delivery from affected IPs to prevent further damage to reputation.
• Hourly volume caps: Setting hourly limits, such as sending no more than 10% of an IP's daily maximum volume in any given hour, helps smooth out sending patterns and is beneficial for reputation.
• Progressive increase rules: Experts advise establishing clear rules for increasing sending throttles, for example, only after 75% of the daily allowed volume has been consistently sent for at least two consecutive days.

Key considerations

• Pre-registration with ISPs: Contacting major ISPs like Microsoft can sometimes lead to new IPs inheriting existing reputation or having higher initial limits, which can significantly accelerate warming. This also relates to best practices for setting email domain connection limits.
• Volume scaling: While complex, a well-managed warming process can enable an IP to send millions of emails daily, demonstrating strong sender reputation.
• Persistent monitoring: Continuous monitoring and adaptation are crucial, as even robust warming strategies can encounter unexpected issues, such as being listed on a blocklist. For more, see an in-depth guide to email blocklists.
• Strategic sending: The initial emails sent during warming should be to highly engaged recipients to establish a positive reputation from the start. This builds trust with ISPs over time.

Key findings

• Gradual volume ramp-up: Documentation consistently advises starting with low email volumes and systematically increasing them over a defined period to build reputation.
• Reputation establishment: The primary goal of IP warming is to build a positive sender reputation with ISPs, which helps ensure emails reach the inbox rather than spam folders.
• Consistency and frequency: Maintaining a steady and consistent sending volume and frequency is often recommended to avoid triggering ISP filters, as sporadic bursts can be seen as suspicious.
• Engagement signals: Sending to highly engaged subscribers initially is crucial for generating positive signals (opens, clicks), which contribute significantly to IP reputation.

Key considerations

• Monitoring and adaptation: While not always explicitly detailing ISP-specific nuances, the underlying message is to monitor deliverability metrics closely and adapt the sending schedule if issues arise. This includes checking for blocklistings and bounces. See our guide on how email blocklists work.
• Authentication: Proper email authentication (SPF, DKIM, DMARC) is a prerequisite for successful warming, as it helps ISPs verify sender legitimacy. For more information, check out a simple guide to DMARC, SPF, and DKIM.
• Patience: Documentation often implies that IP warming is not a quick process and requires patience, typically spanning several weeks to months, depending on volume.
• Subscriber quality: Clean, permission-based email lists are fundamental. Sending to invalid or unengaged addresses during warming can severely hamper progress. Refer to this Mailgun article on domain warm-up for more insights.