Why is my new ESP blocking addresses with an invalid domain block reason when they were delivered by the previous ESP?
Michael Ko
Co-founder & CEO, Suped
Published 8 May 2025
Updated 16 Aug 2025
7 min read
Migrating to a new email service provider (ESP) can sometimes uncover hidden issues with your email list that were previously masked. It can be surprising when addresses that were successfully delivered by your old ESP are suddenly blocked by the new one, especially with a reason like "invalid domain". This situation suggests a shift in how the new ESP validates recipient addresses, which is often a more stringent approach to maintaining their own sender reputation and deliverability standards.
Understanding why this happens is crucial for ensuring your email campaigns reach their intended recipients. The discrepancy usually stems from differences in validation processes, the strictness of domain checks, and how each ESP handles transient DNS issues or non-standard domain configurations. It's a common challenge during ESP migrations, and addressing it effectively requires a deeper look into DNS records and ESP policies.
Understanding the 'invalid domain' block
When an ESP reports an "invalid domain" block, it typically means they attempted to resolve the recipient's domain to find its mail exchange (MX) records, but failed to do so or found an issue. Unlike a hard bounce for an invalid email address (user does not exist), an invalid domain suggests a problem with the domain itself or its DNS configuration. This stricter validation helps the new ESP avoid sending mail to defunct domains, which can negatively impact their sending reputation.
The previous ESP might have had a more lenient validation policy, or perhaps it performed fewer real-time DNS checks before attempting delivery. Some older systems might simply queue the message without robust domain validation, hoping for a later retry or relying on the recipient server to issue a clear bounce. This could mean you had a number of undeliverable addresses that were not being flagged.
The example of a domain like `stgcommercialisti.it` being blocked, even though it appears to have valid MX records, highlights that the issue isn't always straightforward. It suggests the new ESP's specific validation logic might be tripping on something subtle, or it could be an internal misclassification within the ESP's system.
Technical root causes: DNS and email standards
Several technical factors can lead to an "invalid domain" block. The most common is the absence of an MX record for the recipient's domain. An MX record tells sending servers where to deliver mail for that domain. If it's missing or misconfigured, emails can't be routed. Some ESPs also check for an A record, though an MX record is primarily for email. If the recipient domain previously used only an A record for some email services, a new ESP might consider it invalid without an MX record.
Previous ESP behavior
May have performed fewer real-time DNS lookups or relied on the recipient server to explicitly bounce emails. This could lead to a higher reported delivery rate, even for undeliverable addresses.
Their system might have classified some invalid addresses as delivered in their reporting.
New ESP behavior
Likely implements stricter pre-send validation, including robust MX record checks, to maintain its sender reputation. This proactive blocking prevents bounces.
May preemptively suppress recipients whose domains fail these checks, even if an explicit bounce message isn't received from the target mail server. This can impact your email deliverability rate.
Another factor is the domain used in the MAIL FROM (RFC 5321.From) address. Some receiving mail servers, and thus some ESPs, will reject mail if the domain in the MAIL FROM does not have a valid MX record itself. While less common for recipient domain checks, this can occur if the sending domain's DNS is misconfigured.
Sender reputation and list hygiene
Your new ESP, especially if you're using a new dedicated IP, has a strong incentive to maintain a pristine sender reputation from day one. Sending to invalid domains generates hard bounces and can trigger blocklist (or blacklist) placements, harming the IP's new reputation. The previous ESP might have accrued some negative reputation that allowed it to deliver to marginally valid domains, or its older IPs had a higher domain reputation that afforded it more leniency.
This situation often highlights underlying issues with your email list hygiene. It's possible that over time, your list accumulated addresses whose domains became inactive, expired, or had their DNS records removed. While the old ESP might have let these slide, the new ESP's rigorous checks are now surfacing these dormant problems. This can particularly be true if the addresses were acquired a long time ago and haven't been regularly cleaned.
Scenario 1: previous ESP was too lenient
Data Quality: Your previous ESP may have been less aggressive in its pre-send validation or its reporting may have included suppressed addresses as delivered, effectively hiding bounces. This is a common pitfall that can lead to a false sense of good deliverability.
IP Reputation: An old IP with a long history and established reputation might be granted more leniency by ISPs, even when sending to slightly problematic domains. A new, cold IP needs to be strictly validated to build a positive reputation.
Troubleshooting and resolution
The first step is to contact your new ESP's support team. Provide them with specific examples of the blocked addresses and the exact error messages you're receiving, such as "550 5.2.0 mFplpG5Ya3j6emFpmp5Nws dominio non valido / invalid domain". This detailed information will help them investigate their internal logs and identify the precise reason for the block. They may be able to provide insight into their specific validation rules or if it's a misclassification. You may have to refer to this article about resolving deliverability issues when migrating ESPs.
Perform a thorough email list hygiene check. This involves validating every address on your list to ensure the domains are still active and have valid MX records. Services exist that can help identify and remove invalid, inactive, or spam trap addresses. Regularly cleaning your list prevents sending to non-existent domains, which is crucial for maintaining a healthy sender reputation and avoiding blocklists.
If the issue persists for specific domains, manually check their DNS records, especially the MX records. Tools are available to perform DNS lookups for any domain. If you find domains without MX records, those addresses should be suppressed from your sending list. Also, ensure your sending domain itself has a valid MX record, as some recipients and ESPs may check this for the MAIL FROM domain.
Finally, review your email authentication records: SPF, DKIM, and DMARC. While less directly related to "invalid domain" blocks on recipient addresses, misconfigured authentication for your sending domain can still lead to delivery issues. Ensuring your DNS records are correctly set up, as outlined in a guide to DMARC, SPF, and DKIM, is a fundamental step for overall email deliverability.
Views from the trenches
Best practices
Actively maintain and clean your email lists to remove inactive or invalid domains.
Verify DNS records, particularly MX records, for problematic recipient domains.
Communicate proactively with your new ESP to understand their specific blocking policies and get detailed error explanations.
Ensure your own sending domain has correctly configured MX records and email authentication.
Implement a slow ramp-up process for new IPs to build reputation gradually.
Common pitfalls
Assuming past deliverability metrics from an old ESP are accurate indicators for a new one.
Not thoroughly cleaning email lists before migrating to a new ESP.
Ignoring generic block reasons and not seeking clarification from the ESP.
Failing to check the MX records of both recipient and sending domains.
Underestimating the impact of list hygiene on new IP reputation.
Expert tips
Always validate email lists before importing them into a new ESP, particularly if they are older lists.
Understand that different ESPs have varying levels of strictness when it comes to domain validation.
Use bounce reports from your new ESP as a critical tool for identifying list decay and invalid domains.
For problematic domains, consider sending a small test batch to confirm deliverability before a full send.
If an ESP blocks a domain that appears valid, it might be an internal rule or a misclassification requiring direct communication with them.
Marketer view
A marketer from Email Geeks says that the previous ESP might have reported suppressed recipients as delivered, which can create a false impression of success.
2023-04-19 - Email Geeks
Expert view
An expert from Email Geeks states that some ESPs might suppress sends to domains that lack an MX record, preventing email delivery.
2023-04-19 - Email Geeks
Improving your list health
Encountering "invalid domain" blocks when switching ESPs is often a sign that your new provider has more robust validation processes. While frustrating, this can ultimately improve your email deliverability by ensuring you're only sending to truly active and valid addresses. It acts as a necessary clean-up, forcing you to address underlying list hygiene issues that may have gone unnoticed.
By proactively engaging with your ESP, validating your contact lists, and ensuring proper DNS configuration for both recipient and sender domains, you can mitigate these issues. This strategic approach will help your email program thrive on the new platform, leading to better inbox placement and overall campaign performance. Remember, a clean list is a healthy list, and it's a fundamental pillar of strong email deliverability.
