What steps should I take when moving email sending from an ESP to my own infrastructure to resolve unexpected Spamhaus listings and deliverability issues with mailbox providers?
Matthew Whittaker
Co-founder & CTO, Suped
Published 19 Apr 2025
Updated 17 Aug 2025
7 min read
Moving email sending from an Email Service Provider (ESP) to your own infrastructure is a significant undertaking. While it offers more control and potentially cost savings, it also introduces a new set of challenges, especially concerning sender reputation and deliverability. We've seen many organizations encounter unexpected issues, such as immediate Spamhaus listings and rejections from major mailbox providers, even before sending production traffic.
The transition process isn't always straightforward. Unlike using an ESP with established sender reputation, building your own can feel like starting from scratch. Mailbox providers and blocklist operators often treat new infrastructure with extreme caution, leading to deliverability problems if not managed correctly. This guide outlines the key steps to navigate these challenges and resolve common issues like unexpected blocklistings and email rejections.
The unexpected start: new domains and ip reputation
When transitioning to your own email infrastructure, you might be surprised to find new domains or IPs (Internet Protocols) listed on blocklists like the Spamhaus DBL (Domain Blocklist) before a single email has been sent. This can be alarming, but it's often a preventative measure by some blocklist providers. They may flag newly registered domains as suspicious due to the historical pattern of spammers quickly registering and abandoning domains. The listing might be temporary and could be lifted after a period, usually a month, provided no malicious activity is detected. It's important to understand that such listings are a factor, not an absolute rule, but they still impact initial deliverability.
Beyond domain age, mailbox providers (MBPs) assess a wide range of factors to determine sender reputation. This includes the reputation of your IP addresses, even new ones. I've observed scenarios where IPs that weren't on any public blocklists were still rejected by major MBPs like Microsoft and Apple. These rejections often point to internal (private) blocklists or reputation systems that consider factors beyond typical public blocklist checks.
For instance, I've seen bounce messages explicitly stating an IP address is on an internal blocklist, even when external checkers showed it as clean. This suggests that mailbox providers might be evaluating broader reputation signals, such as the reputation of the entire IP subnet or Autonomous System (AS) where your servers reside. An IP block for an entire network, even if you are just one small part of it, can significantly hamper your email deliverability right from the start.
Example Outlook Bounce Message
550 5.7.1 Unfortunately, messages from [YOUR IP] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140).
Beyond IPs and domains: other reputation factors
When moving from an ESP, many subtle reputation signals come into play beyond just IP and domain. Mailbox providers build a comprehensive profile of your sending entity. This includes your Autonomous System (AS), the registrar of your domain, the CIDR (Classless Inter-Domain Routing) block your IPs belong to, your hosting company, and even your DNS provider. Each of these elements contributes to your overall sender trust.
Your DomainKeys Identified Mail (DKIM) key, for example, is another reputation holder. If the previous owners of your newly acquired IPs or IP ranges (CIDR blocks) had a history of sending spam, this pre-existing negative reputation can carry over and affect your deliverability, regardless of your clean sending practices. This is often the case when a large hosting provider is associated with a history of abuse. Even if you haven't sent a single email, your network neighborhood could be causing initial rejections.
These hidden factors highlight why relying solely on public blocklist checks isn't sufficient. Mailbox providers maintain sophisticated internal systems that aggregate data from numerous sources. Understanding this broader landscape of reputation is crucial for effective deliverability management.
Factor
Impact on Deliverability
Checking Mechanism
IP Subnet/AS Reputation
Inherited reputation from the hosting provider or network block, even for new IPs.
Cisco Talos Intelligence, internal MBP systems.
Domain Registration Age
New domains are treated with suspicion by some blocklists and MBPs initially.
Mismatch between HELO hostname and reverse DNS can trigger spam filters.
MXToolbox or similar tools.
Strategic migration and warm-up
When migrating from an ESP to your own infrastructure, a common challenge is deciding whether to reuse existing sending domains or establish new ones. If your current domains have a strong, positive reputation, it's almost always beneficial to retain them. This allows you to bootstrap your new IP reputation with a known good domain history. Changing subdomains (e.g., from emails.yourdomain.com to email.yourdomain.com) can still be perceived as a new sender by some MBPs, as their reputation systems might key off the exact subdomain or even the underlying IP address associated with that subdomain.
The core of successful migration lies in the IP warm-up process. This means gradually increasing your sending volume over time to build a positive reputation with MBPs. Starting with a low volume of highly engaged recipients and slowly ramping up allows providers to learn about your sending patterns and validate your legitimacy. Simultaneously sending from multiple datacenters with new IPs can look suspicious if not managed meticulously, as it resembles tactics used by spammers. Consider using multiple datacenters for redundancy or geographic distribution, but carefully manage the reputation warming for each set of IPs.
Old ESP approach
Shared reputation: Leveraging the ESP's established IP and domain reputation.
Managed warm-up: ESP handles the gradual increase in sending volume.
New self-hosted approach
Building reputation: Requires careful IP and domain warm-up.
Full DNS control: Manual setup of all authentication records (SPF, DKIM, DMARC).
Strategic sending: Careful segmentation and volume control to avoid flags.
Navigating postmaster pages and continuous monitoring
Once you've confirmed your technical setup is correct and you're embarking on the warm-up, continuous monitoring and proactive communication with mailbox providers are essential. Many issues, particularly with new IPs, can be resolved by directly engaging with the postmaster teams of the affected MBPs.
You'll often find resources on their dedicated postmaster pages, such as Yahoo's Postmaster site, to submit de-listing requests or inquire about specific rejection errors. Provide as much detail as possible about your sending practices and setup. While some MBPs might be slower to respond than others, consistent and clear communication is vital for recovering from initial blocks and establishing a solid reputation.
Important: contact mailbox providers
Even if your IPs aren't on public blocklists, individual mailbox providers (MBPs) can still block or filter your emails based on their internal reputation systems. If you're seeing rejections, particularly from major providers like Apple or Outlook, your best course of action is to:
Review bounce messages: They often contain codes or URLs that direct you to the specific reason for the block.
Check postmaster sites: Most MBPs offer tools and forms for senders to request de-listings or assistance.
Provide context: Clearly explain your migration, sending volume, and commitment to best practices.
Crucially, remember that deliverability issues are rarely solely an infrastructure problem. They frequently stem from underlying list management and hygiene practices. An ESP often acts as a buffer, masking issues that become apparent when you run your own servers. Focusing on your user acquisition process, explicit consent for sending, and the quality and relevance of your email content are paramount. These factors ultimately dictate your long-term sender reputation and inbox placement.
Concluding thoughts on self-hosting email
Migrating email sending from an ESP to your own infrastructure is a complex transition that demands meticulous planning and ongoing vigilance. Initial challenges, such as unexpected Spamhaus blocklistings or rejections from major mailbox providers, are not uncommon but can be effectively addressed with a structured approach. It's about much more than just setting up servers; it's about building and maintaining trust with the entire email ecosystem.
By understanding the nuanced factors that contribute to sender reputation, proactively engaging with mailbox providers, and prioritizing consistent, high-quality sending practices, you can navigate this transition successfully and achieve optimal email deliverability on your self-managed infrastructure. We've compiled a list of common questions below to help you further.
Views from the trenches
Best practices
Start your IP warm-up process slowly and gradually, increasing volume over weeks or months.
Maintain consistent sending patterns once IPs are warmed up to sustain good reputation.
Implement strong email authentication: SPF, DKIM, and DMARC are non-negotiable.
Regularly monitor your IPs and domains on public and private blocklists.
Common pitfalls
Not thoroughly warming up new IPs before sending high volumes, leading to immediate blocks.
Failing to set up proper reverse DNS (rDNS) records for your sending IPs.
Ignoring bounce messages and error codes, which provide crucial diagnostic information.
Assuming a clean public blocklist status means you're not on any private MBP blocklists.
Expert tips
DNS provider choices and hosting company reputation can impact your deliverability.
Your existing domain reputation can help bootstrap new IP reputation, and vice-versa.
Deliverability issues are often rooted in list management and audience engagement, not just infrastructure.
Yahoo and Gmail often accept initial emails, which helps in building a reputation with engagement data.
Marketer view
Marketer from Email Geeks says a new network starting to send emails from two separate data centers at the same time is completely normal, and some providers may reject these emails during initial warm-up rounds.
2023-09-29 - Email Geeks
Expert view
Expert from Email Geeks says that while Spamhaus occasionally makes mistakes, they generally follow good policy and are helpful to non-spammers regarding listings. Engineering decisions seem plausible.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
Cisco Talos Intelligence, internal MBP systems.
MXToolbox or similar tools.
Apple or 
Outlook, your best course of action is to: 
