How does email reputation transfer during IP warming with a new ESP, and how to resolve deliverability issues?

Key findings

• Reputation components: Sender reputation is assessed by mailbox providers using complex algorithms that consider factors like the sending IP, the authenticated domain, and often the DKIM selector, forming a reputation ip:selector:domain tuple.
• Domain vs. IP reputation: While your parent domain's reputation may partially transfer if it remains consistently authenticated (e.g., via DKIM) with the new ESP, the new IP and DKIM selector will need to build their own reputations from scratch.
• ISP variations: Different Internet Service Providers (ISPs), such as Google, Outlook (Oath), and Microsoft, utilize distinct machine learning algorithms for reputation assessment, meaning deliverability can vary across them even for the same sending practices.
• Engagement data: Pulling lists of recently engaged subscribers (e.g., opens or clicks within 60 days) from your old ESP is a sound strategy to initiate warming, as these recipients are more likely to interact positively with your emails and contribute to building a good reputation on the new IP.

Key considerations

• IP re-warming: An IP that was previously warmed but then remained dormant for several months will likely need to be re-warmed to re-establish its sending trust. This is a common deliverability issue during new IP and subdomain warming. Common deliverability issues during new IP warming can include elevated spam complaints and bounces.
• Monitoring spam rates: A spam rate, even as low as 0.2% on Google Postmaster Tools, can indicate underlying deliverability issues, particularly with Gmail. Regular monitoring is essential.
• Diagnostic testing: To pinpoint the exact cause of deliverability problems, isolate variables by performing seed testing. This helps determine if the issue stems from IP reputation, domain reputation, or specific email content (e.g., tracking pixel URLs). More on how to warm up an IP address is available in this guide.
• Strategic list segmentation: Focus on sending to your most engaged segments first during warming. While this may reduce initial send volumes, it significantly improves deliverability prospects and helps in email frequency and volume management after IP warming.

Key opinions

• Initial warming strategy: Many marketers opt to start IP warming by sending emails only to subscribers who have recently opened or clicked, assuming this segment represents the highest engagement and will positively influence the new IP's reputation.
• Re-warming after dormancy: There's a consensus that an IP address, once warmed, loses its established reputation if it remains inactive for an extended period, necessitating a re-warming process.
• ISP-specific deliverability: Marketers often observe inconsistent deliverability, with emails landing in spam at one major ISP (e.g., Gmail) while performing well at others, even within the same warming phase.
• Spam rate interpretation: A low spam rate, such as 0.2%, is still considered a flag for concern, indicating that a portion of the audience is actively marking emails as unwelcome, potentially due to segmentation issues.

Key considerations

• Audience segmentation: When issues arise, especially high spam complaints, marketers should critically examine their audience segmentation. Understanding how internet service providers track email engagement can shed light on why certain groups are complaining.
• Leveraging existing engagement: While clicks are ideal for identifying highly engaged users, marketers often expand to include opens to ensure a sufficient volume for warming, even if it means a larger, less hyper-engaged list.
• Balancing migration and issue resolution: It's a delicate balance to continue with an ESP migration while simultaneously addressing historical deliverability issues. Prioritizing the most impactful problems, like major ISP spam folder placement, is key.
• Ongoing testing: Using seed list testing tools, even those that show nuances like Gmail being fine but Google Inbox being spam placed, provides actionable insights for adjustment.

Key opinions

• Algorithm complexity: There is no definitive answer on reputation transfer due to the proprietary and constantly evolving machine learning algorithms used by major mailbox providers like Google, Oath, and Microsoft.
• Reputation tuple: ISPs often build reputation on a unique combination of the sending IP, DKIM selector, and domain (the ip:selector:domain tuple). A change in IP or selector will create a new tuple that needs to establish its own reputation, though domain reputation persists.
• Domain reputation stability: If emails are consistently DKIM-signed with the same parent domain, a portion of the domain's reputation is likely to carry over to the new ESP and IP, offering some continuity.
• Engagement type impact: The specific type of subscriber engagement (e.g., replies versus opens) can influence which aspect of reputation (e.g., the From address) is primarily affected.

Key considerations

• Strategic seed testing: To diagnose deliverability issues, experts recommend comprehensive seed testing, particularly with problematic ISPs like Gmail, to isolate whether the issue is related to IP, DKIM, or domain reputation. This helps in diagnosing email deliverability issues.
• Volume ramp-up: Gradual increases in sending volume through the new ESP are crucial for proper IP warming. This process, along with sending to the most engaged subscribers first, forms the bedrock of a successful warm-up.
• List hygiene integration: The transition to a new ESP or IP warming phase is an ideal opportunity to perform rigorous list hygiene, purging unengaged subscribers to improve overall sending health. This helps to recover email domain and IP reputation if issues occur.
• Hidden reputation factors: Experts warn that even the presence of a domain name in unexpected places, such as a tracking pixel URL, can influence its reputation if that domain is associated with spam.

Key findings

• Warming definition: IP warming is the process of gradually increasing email volume sent through a new IP address over a period to build its sending reputation and trust with recipient servers.
• Reputation factors: IP reputation is largely determined by email volume, content relevance, subscriber engagement (opens, clicks), and crucially, complaint rates and bounce rates.
• Shared vs. dedicated IPs: For shared IPs, the ESP typically handles warming and reputation management for the collective pool. With dedicated IPs, the sender assumes direct responsibility for this process.
• Domain's role: Your sender reputation is tied to both your IPs and your sending domains, with domain warming also being a critical strategy for establishing trust.

Key considerations

• Trust building: Warming helps mailbox providers evaluate and build trust in your new sending identity, which is crucial for preventing messages from landing in spam or being blocked. This is a core part of email deliverability in 2025.
• Preventative measures: Proper IP and domain warming is designed to prevent deliverability issues such as messages being routed to spam folders, experiencing high bounce rates, or being outright blocked by recipient servers.
• Data migration: When switching ESPs, safely and efficiently transferring all relevant data, including email content, templates, and especially lists of bounces, complaints, and unsubscribes, is paramount to maintain reputation.
• Monitoring tools: Utilize tools like Google Postmaster Tools for tracking spam placement during IP warming. Understanding how Amazon SES guides on IP warming can be beneficial.