Why is my new ESP blocking addresses with an invalid domain block reason when they were delivered by the previous ESP?

Key findings

• Stricter validation: New ESPs often employ more stringent validation rules than previous providers, leading to blocks on domains that might have been accepted before. This can include checks for specific DNS records or real-time connectivity.
• IP reputation impact: When moving to a new ESP, particularly with a new dedicated IP, your sender reputation might be starting fresh or be influenced by the IP's prior history, affecting initial deliverability.
• Preemptive suppression: Some ESPs may preemptively suppress (or block) recipient addresses based on internal heuristics, even before attempting delivery to the recipient's mail server. This can lead to an invalid domain block reason without an actual bounce from the receiving end.
• DNS record verification: The new ESP might be performing more thorough checks for valid MX records or other DNS configurations for both the recipient domain and (less commonly) the sender's from domain.
• Deliverability reporting variations: Different ESPs have varying reporting standards; some may count suppressed or blocked addresses as delivered in their metrics, masking underlying issues. Always review your ESP's bounce and block notifications carefully.

Key considerations

• ESP internal logic: It is crucial to engage directly with your new ESP to understand their specific classification logic for invalid domain blocks. They can often provide more detailed insights into why a particular domain is being flagged.
• Recipient domain checks: Verify that the blocked recipient domains genuinely have active MX records and are configured to receive email. Small, private domains can sometimes have unusual or temporary DNS setups.
• Sender domain authentication: Ensure your sending domain (the from address domain) has a proper MX record, even if it is not typically used for receiving mail directly. Some receiving servers may check this as a basic spam prevention measure.
• List hygiene: While a past ESP might have been lenient, the new ESP may be enforcing stricter list hygiene. Regularly clean your email lists to remove invalid or unengaged addresses. This is critical for maintaining a healthy sender reputation.

Key opinions

• ESP variance: Marketers frequently observe that different ESPs have varying approaches to validating recipient domains, leading to inconsistencies in deliverability. What was once accepted might now be blocked.
• New IP reputation: The use of a new dedicated IP, even if warmed up, can result in different blocking patterns as its reputation is still being established with various ISPs.
• Recipient domain checks: Concerns are raised about whether the recipient domains genuinely lack proper MX records, or if the new ESP is simply being more aggressive in its pre-delivery checks.
• Misclassification by ESP: There's a strong sentiment that some ESPs might misclassify valid domains as invalid due to internal system quirks or overly broad filtering rules.
• Reporting discrepancies: Marketers are wary that previous ESPs might have reported suppressed recipients as delivered, which would mask underlying deliverability issues that only become apparent with a new, more transparent provider.

Key considerations

• Direct ESP communication: The primary recommendation is always to contact the new ESP's support to gain clarity on their specific invalid domain block reasons and internal processes.
• Validate recipient domains: For specific problematic domains, conduct independent checks (e.g., using MX lookup tools) to confirm their existence and mail-receiving capabilities.
• Review list acquisition: Re-evaluate how the email addresses were acquired. Poor list acquisition can lead to issues that surface with a more scrutinizing ESP. Regularly removing invalid addresses is crucial for good email deliverability.
• Sender domain health: Ensure that your own sending domain (the one in the From header) has a properly configured MX record, as some receiving servers may check this.

Key opinions

• MX record presence: Some ESPs or receiving mail servers will suppress or reject emails if the recipient's domain does not have a properly configured MX record in its DNS, even if an A record exists.
• ESP misclassification: The issue is often a misclassification by the new ESP rather than a genuine problem with the recipient domain, necessitating direct engagement with the ESP.
• Sender domain validation: Receiving servers may check for an MX record on the sender's From domain (RFC 5321.From or RFC 5322.From) as a spam prevention measure, rejecting mail if one is absent.
• Preemptive vs. rejection: It's important to differentiate if the ESP is preemptively suppressing the address or if the block reason is derived from a hard bounce (rejection) by the recipient's MX server.
• Previous ESP reporting: A key consideration is that the previous ESP might have included suppressed or invalid recipients in their delivered reports, masking the true deliverability status of those addresses.

Key considerations

• Engage ESP support: The most effective step is to contact the new ESP to understand their specific invalid domain definition and what criteria trigger this block reason for specific addresses. This can help diagnose and fix deliverability issues.
• Verify recipient domains: Independently verify the MX records of the blocked recipient domains. Tools can confirm if a domain is genuinely configured to receive mail.
• IP warming strategy: For new dedicated IPs, ensure a proper IP warming strategy is in place. Initial low volume or problematic sends can negatively impact early reputation, leading to stricter filtering by ISPs.
• Header review: Examine the full SMTP response for the invalid domain blocks. The actual error message (e.g., 550 5.2.0 invalid domain) can reveal whether the block originated from the recipient's server or the ESP's internal system. This is a crucial step when you are facing IP or domain blocklist issues.
• Sender policy enforcement: Implement and maintain strong email authentication (SPF, DKIM, DMARC) for your sending domains. This improves your sender reputation and reduces the likelihood of legitimate mail being flagged.

Key findings

• MX record necessity: RFC 5321 (SMTP) mandates that a recipient domain must have an MX (Mail Exchange) record to specify where its mail should be delivered. The absence or misconfiguration of this record often leads to invalid domain rejections.
• RFC compliance: Modern ESPs and receiving mail servers increasingly enforce compliance with RFCs. This includes checking not only the recipient domain's MX but also potentially the sender's (envelope From/RFC 5321.From) domain for valid MX records to prevent backscatter and unauthorized sending.
• Domain authentication: Proper implementation of SPF, DKIM, and DMARC is critical. While not directly about invalid domains, a lack of authentication can lead to emails being filtered or blocked by strict recipient policies.
• Spam trap avoidance: Documentation often advises against sending to old or unverified lists, as these may contain spam traps, including those on domains that no longer accept mail, which can damage sender reputation and trigger blocklisting (also known as blacklisting).

Key considerations

• DNS health check: Before migration, perform a comprehensive DNS health check for both your sending domains and frequently used recipient domains to identify any missing or incorrect MX records.
• Validate list before sending: Utilize email validation services to clean your list and remove addresses associated with invalid or problematic domains before sending, especially when moving to a new ESP. This is a common practice to avoid being placed on an email blocklist.
• Monitor SMTP responses: Pay close attention to SMTP bounce codes. A 550 5.2.0 invalid domain indicates a hard bounce, meaning the receiving server explicitly rejected the mail. This differs from an ESP's internal suppression.
• IP warming rigor: Follow a diligent IP warming schedule. A cold IP with a sudden surge in problematic sends (even to what appear to be valid domains) can trigger strict filtering or blocks by ISPs, as outlined in postmaster guidelines.
• Understanding RFCs: Familiarize yourself with relevant RFCs and industry best practices for email sending, as new ESPs are more likely to enforce these standards rigorously.