How to resolve email deliverability issues and IP blocks when migrating to a new ESP?

Key findings

• Identify cause: Do not blindly resend blocked emails; instead, investigate the root cause of the block to prevent further issues.
• Resending risk: Resending previously rejected messages from a different ESP or domain can be perceived as spamming and trigger snowshoe detection, a technique where spammers use multiple domains or IPs to bypass filters.
• Microsoft blocks: Microsoft frequently blocklists (or blacklists) IPs due to a history of abusive senders on a network, especially within shared IP pools, as outlined in their troubleshooting documentation.
• ESP responsibility: Your new ESP should actively assist in resolving IP blocks, as it is generally their network's reputation that is at stake and they have the direct channels to mailbox providers.
• Shared IP blocks: While some filtering may occur, actual IP blocklists are less common on *new* shared IPs if the ESP manages its pools well. Persistent blocks often point to existing bad actors on that shared IP.

Key considerations

• Migration strategy: Plan a gradual migration to a new ESP, implementing proper IP warming procedures. This is crucial for maintaining sender reputation and avoiding deliverability issues from the outset, as detailed in guides like the AWS guide to IP and domain warming.
• Monitor bounce codes: Pay close attention to specific bounce messages (e.g., the 550 5.7.1 Microsoft block) to understand the exact nature of the issue. These codes provide valuable diagnostic information.
• ESP support: Leverage your new ESP's support team to address IP blocklistings and understand their approach to managing shared IP pools. Your ESP is your primary resource for resolving network-level issues.
• Authentication: Ensure all email authentication protocols (SPF, DKIM, DMARC) are correctly configured for your new ESP. Incorrect setup can lead to IP blacklisting or filtering.
• List hygiene: Clean your email list before migrating to avoid sending to invalid or inactive addresses, which can negatively impact your sender reputation and lead to more blocks.

Key opinions

• Avoid blind resends: Most marketers agree that simply resending blocked emails without investigation is detrimental and can significantly hurt sender reputation.
• Understand the why: It is crucial to diagnose the specific reason for the block before taking any action, as bounce codes provide vital clues.
• Snowshoeing risk: Marketers are aware that sending the same content from multiple ESPs (especially during migration) can trigger snowshoe detection, leading to more widespread blocks.
• ESP responsibility: Many believe that the ESP should actively manage and resolve IP blocklistings, rather than solely waiting for reputation to improve over time.
• Authentication is key: Properly configured SPF, DKIM, and DMARC are often cited as critical for successful sending, particularly from shared IP pools.

Key considerations

• Communication with ESP: Marketers emphasize the need for clear communication with the new ESP regarding deliverability issues and their resolution process, as suggested by Dotdigital's migration checklist.
• Patience and monitoring: While frustrating, a degree of patience coupled with diligent monitoring of bounce rates and blocklists is advised during the warming period.
• Gradual ramp-up: Stick to a cautious IP warming schedule even if initial blocks occur, resisting the urge to blast emails, which can damage reputation further. Learn more about retaining sender reputation during ESP migration.
• Impact of shared IPs: Be aware that issues on shared IP pools can stem from other senders using the same IP, affecting your deliverability through no fault of your own.
• Authentication protocols: Ensure that your SPF, DKIM, and DMARC records are properly configured for your new sending environment, as emphasized by Dyspatch's guide to switching ESPs.

Key opinions

• Diagnose before acting: Experts strongly advise against resending blocked emails without first understanding the precise reason for the block, as this can worsen reputation.
• ESP responsibility for shared IPs: A common expert view is that the ESP should proactively manage shared IP pool reputation and resolve blocks, as it is their network that is being impacted.
• Shared IP vs. dedicated IP blocks: Blocks are less expected on shared IPs during warm-up compared to dedicated IPs, suggesting an underlying issue with the shared pool or network management.
• Authentication critical: Proper DKIM signing and whitelisting of return paths are considered essential for deliverability from shared ESP IP pools, as these verify sender legitimacy.
• "Hope" is not a strategy: Relying on automatic "upgrades" to a better IP pool is not an effective solution for persistent blocks; active intervention is often required.

Key considerations

• Specific block messages: Always provide the exact bounce message to your ESP, as it contains critical information for diagnosis and helps in understanding how to diagnose email deliverability issues.
• Shared IP quality: Investigate the quality and management practices of the shared IP pools offered by the new ESP. A poorly managed pool can severely impact your sending.
• Proactive ESP engagement: Push your ESP to actively address IP blacklistings rather than accepting vague promises of future improvement. Use blocklist monitoring to keep them accountable.
• Authentication configuration: Verify that SPF, DKIM, and DMARC records are correctly set up and aligned for the new sending infrastructure. Incorrect setup is a common cause of deliverability failures.
• Reputation impact: Understand that moving to a new ESP involves establishing new sender reputation, which requires careful management and adherence to best practices, as Word to the Wise often explains.

Key findings

• IP warming protocols: Most documentation emphasizes the necessity of gradual IP warming to build trust and reputation with receiving mail servers, as a sudden surge can trigger spam filters.
• Sender authentication: SPF, DKIM, and DMARC are universally recommended for verifying sender identity and preventing spoofing, playing a crucial role in deliverability.
• Reputation metrics: Mailbox providers use various metrics, including spam complaints, bounce rates, and engagement, to assess sender reputation, which directly impacts inbox placement.
• Blocklist criteria: IP addresses can be blocklisted (or blacklisted) for various reasons, including sending unsolicited mail, consistently high complaint rates, or being part of a compromised network.
• Troubleshooting tools: Providers often offer postmaster tools or troubleshooting guides to help senders diagnose and resolve deliverability issues, providing transparency into their filtering.

Key considerations

• Adhere to standards: Ensure your email sending infrastructure and practices comply with RFCs and industry best practices. This includes proper implementation of SPF, DKIM, and DMARC.
• Monitor postmaster tools: Regularly check tools like Google Postmaster Tools and Outlook.com Postmaster for insights into your sending reputation and potential issues.
• Proper DNS records: Verify that all necessary DNS records (SPF, DKIM, DMARC, reverse DNS) are correctly published and maintained for your new sending domain, as highlighted by Klaviyo's documentation on migrating sending domains.
• Handle bounces correctly: Implement proper bounce handling to remove invalid addresses from your list and avoid sending to non-existent users, which can degrade reputation.
• Feedback loops: Sign up for feedback loops (FBLs) where available to promptly identify and remove users who mark your emails as spam. This is crucial for maintaining list hygiene.