Where can I find a list of all mailbox providers and their domains?

Key findings

• Dynamic landscape: The email domain landscape is constantly evolving due to corporate acquisitions, mergers (for instance, Cox becoming part of Yahoo), and legacy domains.
• Provider complexity: Major mailbox providers often host thousands of domains, making it impractical to maintain a single, comprehensive list. Even if a user accesses their email through one major provider's web interface, there might be additional layers of filtering or different backend systems involved, such as with certain AT&T domains routed through Yahoo.
• No complete list exists: There is no single, publicly available list that is truly exhaustive or consistently current. Online resources often become outdated quickly.
• Focus on identification methods: Instead of a static list, a more effective approach involves using tools and methods to dynamically identify the mailbox provider associated with an email address or domain, typically through DNS records (MX records).

Key considerations

• Identifying providers dynamically: For email deliverability professionals, understanding how to identify the mailbox provider for a given domain is more crucial than relying on outdated lists. This often involves looking up MX records to determine the email server responsible for a domain.
• Importance for deliverability: Knowing the mailbox provider is essential for optimizing email deliverability, as each provider (e.g., Gmail, Outlook) has its own filtering rules, reputation metrics, and best practices. You can learn more about main domain names used by large free email providers.
• Manual verification: When a specific email provider needs to be identified, especially for troubleshooting, directly querying DNS records for the domain's MX (Mail Exchange) record will reveal the responsible server. This is a common practice in deliverability troubleshooting.
• Leveraging existing knowledge: While a complete list is elusive, resources that list top email service providers can provide a starting point for understanding the major players in the market.

Key opinions

• Desire for complete lists: Marketers frequently express a desire for exhaustive lists of MBPs and their domains to inform their email strategies.
• Outdated information: Existing online lists are often found to be incomplete or quickly outdated, leading to frustration.
• Overwhelming number of domains: The sheer volume of domains managed by major providers makes the idea of a comprehensive list seem daunting, if not impossible.
• Need for dynamic solutions: Many recognize that building their own dynamic system that evolves with each send might be the only feasible way to track domain ownership.

Key considerations

• Adapting to change: Marketers must adapt to the fluidity of domain ownership, such as when ISPs merge or are acquired, affecting how emails are routed and filtered. This can impact average email deliverability rates.
• Understanding filtering layers: Even if a domain is associated with a major MBP, there might be additional layers of filtering or specific policies that affect deliverability. Marketers must understand these nuances to avoid issues like emails going to spam.
• Prioritizing major providers: While a full list is unachievable, focusing on the major MBPs like Gmail, Outlook, and Yahoo, and their primary associated domains, provides a good baseline for most marketing efforts.
• Resource utilization: Marketers often refer to resources that compile information on popular email providers to guide their understanding, even if these lists are not exhaustive for all domains.

Key opinions

• Incompleteness of lists: Experts firmly state that no list of domains can ever be truly complete, given that major providers alone serve thousands of domains.
• Challenges of dynamic identification: Attempts to create real-time identification systems by querying SMTP banners can lead to issues, including being flagged as abusive by some ISPs.
• DNS-based identification: The preferred and safer method for identifying mailbox providers is via DNS lookups, which determine who answers for a domain's mail exchange.
• Corporate offerings and legacy domains: The complexity is further compounded by corporate email offerings and the absorption of legacy domains by major players like Google, Microsoft, and Yahoo.

Key considerations

• Safeguarding sender reputation: It is crucial to use non-intrusive methods like DNS queries for domain identification to avoid triggering abuse reports, which can negatively impact sender reputation. Learn more about how domain reputation is measured.
• Continuous adaptation: Deliverability professionals must continuously adapt their strategies as mailbox provider networks evolve, impacting how emails are handled. This ongoing process helps boost email deliverability rates.
• Focus on engagement: Experts recommend focusing on recipient engagement and sender reputation metrics over static domain blocklists or blacklists, as these are more reliable indicators of inbox placement.
• Authoritative resources: Referencing authoritative resources like Spam Resource can provide valuable insights into ISP domains and deliverability best practices, even if they don't offer a complete list.

Key findings

• Decentralized nature: Email infrastructure is decentralized, meaning there isn't one central authority or database that maintains a list of all mailbox providers and their domains.
• DNS as the backbone: The Domain Name System (DNS) and Mail Exchange (MX) records are fundamental for routing email, allowing systems to dynamically discover which server handles mail for a given domain.
• Protocol focus: Documentation emphasizes the protocols (e.g., SMTP) and standards (e.g., RFCs) that govern email communication, rather than listing specific entities. This ensures interoperability across diverse providers.
• Domain ownership and management: Technical documentation explains how domain owners configure their MX records to point to a chosen email service provider, defining who their mailbox provider is. This is outlined in resources defining an email domain name.

Key considerations

• Understanding email authentication: Familiarity with authentication protocols like SPF, DKIM, and DMARC is critical, as these are how mailbox providers verify sender legitimacy, irrespective of their specific domain. A simple guide to DMARC, SPF, and DKIM can be helpful.
• Monitoring ISP changes: While static lists are not maintained, various resources exist that identify significant changes or updates from major ISPs that affect email deliverability. This knowledge is important for identifying ISP changes.
• Technical evaluation criteria: When considering email providers, technical documentation often evaluates them based on security features, storage capacity, and user interface, rather than merely domain listings.
• Importance of domain reputation: Technical documentation from mailbox providers frequently details how they assess domain reputation, which directly impacts inbox placement, highlighting the dynamic nature of how domains are perceived.