Summary
To significantly improve email deliverability and mitigate anti-phishing warnings, a dual approach focusing on robust email authentication and methodical subdomain warm-up is essential. Proper configuration of SPF, DKIM, and DMARC for your domain and any sending subdomains verifies sender identity, prevents spoofing, and instructs receiving servers on how to handle unauthenticated mail. Concurrently, new subdomains require a gradual warm-up process, incrementally increasing sending volume to engaged recipients, which builds a positive sending reputation with mailbox providers and helps avoid immediate flagging by spam filters and anti-phishing systems.
Key findings
- Authentication cornerstone: SPF, DKIM, and DMARC are fundamental for verifying sender identity, preventing spoofing, and significantly reducing phishing attempts. DMARC, especially with "p=reject" or "p=quarantine", serves as a primary defense against email fraud.
- Subdomain reputation management: Each subdomain used for sending email effectively starts with a fresh sending reputation. This necessitates a proper warm-up to build trust with internet service providers (ISPs); without it, high volumes from new subdomains can trigger spam filters and anti-phishing systems, leading to blocklisting or poor inbox placement.
- Alignment is key: For DMARC to pass and to prevent anti-phishing warnings, the domain in the "From" header (visible to the user) must align with the domain authenticated by SPF or DKIM. This principle applies equally to subdomains, with "relaxed alignment" often being a recommended approach, particularly when using third-party senders.
- Phishing warning triggers: Anti-phishing warnings commonly arise from unauthenticated emails, poor sending reputation, or new/high-volume sending domains. While Microsoft's anti-phishing headers sometimes target 'cousin domains' (similar-looking domains), the underlying cause remains a lack of sender trust.
- Reputation segregation: Using subdomains offers the strategic advantage of segregating your sending reputation. This means deliverability issues on one type of email stream (e.g., transactional) will not negatively impact the reputation of another subdomain used for different purposes (e.g., marketing emails).
Key considerations
- Configure dns records meticulously: For each sending subdomain, configure separate DNS records for SPF (a TXT record specifying authorized IPs) and DKIM (typically a CNAME record pointing to a public key). DMARC should generally be implemented at the organizational domain level to cover subdomains, ensuring proper SPF and DKIM alignment.
- Implement dmarc policies: Set a DMARC policy of "p=quarantine" or "p=reject" to instruct receiving mail servers to treat unauthenticated emails as suspicious, thereby reducing the likelihood of legitimate emails being mistaken for phishing attempts.
- Execute a methodical warm-up: Gradually increase sending volume for new subdomains over several weeks or months, starting with small batches (e.g., 50-100 emails per day) to highly engaged recipients. This signals legitimate sending behavior and helps build a positive reputation with mailbox providers.
- Monitor deliverability metrics: Constantly track key metrics such as bounce rates, open rates, and spam complaints during the warm-up period. Adjust sending volume and strategy based on these metrics to maintain a good reputation and prevent issues.
- Maintain consistency: Ensure consistent sending volume and frequency throughout the warm-up process, avoiding sporadic sending or sudden large spikes. These can negatively impact reputation and trigger anti-phishing warnings.
What email marketers say
8 marketer opinions
Building on the foundation of robust email authentication, effectively managing subdomains for email sending involves understanding alignment nuances and executing a disciplined warm-up strategy. While anti-phishing warnings often target 'cousin domains' (similar-looking domains) more than subdomains, proper configuration, especially with relaxed DKIM alignment, is crucial, particularly when leveraging third-party sending services. For any new subdomain, a gradual warm-up is paramount, as each starts with a clean slate regarding sender reputation. This methodical approach of increasing volume to engaged recipients not only fosters a positive standing with internet service providers (ISPs) but also provides a distinct advantage by isolating sending reputations, thereby minimizing the risk of deliverability issues or anti-phishing flags affecting other email streams.
Key opinions
- Dkim alignment nuances: Relaxed DKIM alignment is frequently recommended over strict alignment, particularly when integrating third-party sending services. This approach facilitates proper DMARC alignment and generally improves deliverability.
- Conditional warm-up necessity: A dedicated warm-up process for a new subdomain may not be necessary if an existing, healthy, DKIM-signed mail stream already maintains the same 'd=' (DKIM domain) for the new sending activities, as the reputation is largely carried over.
- Engagement-led warm-up: Prioritizing sending to highly engaged subscribers during the initial warm-up phases is critical for rapidly building a positive sending reputation. High engagement signals legitimacy to ISPs, contributing to better inbox placement.
- Consistency in warm-up builds trust: Maintaining consistent sending volume and frequency, while avoiding sporadic sending or sudden, large spikes, is paramount for gradually building trust with mailbox providers and preventing reputation damage or being added to a blacklist.
- Strategic reputation segregation: Utilizing subdomains provides a powerful mechanism to isolate sending reputations. This ensures that deliverability issues on one email stream (e.g., transactional emails) do not negatively impact the reputation of other subdomains used for different purposes (e.g., marketing emails).
Key considerations
- Optimize dkim alignment: When configuring email authentication, especially with third-party senders, aim for relaxed DKIM alignment. This means ensuring the '821From' (return-path) uses a subdomain while the 'd=' value (DKIM domain) aligns, which helps prevent anti-phishing warnings and ensures DMARC compliance.
- Implement a phased warm-up schedule: For new subdomains, establish a methodical warm-up plan. Start with a very low daily volume, perhaps 50-100 emails, and gradually increase it over several weeks or even months. This progressive approach builds trust with internet service providers (ISPs).
- Prioritize engaged recipients for warm-up: Initiate the warm-up process by sending to your most active and engaged subscribers. High open and click rates signal positive sending behavior to ISPs, which rapidly improves the subdomain's reputation and helps avoid being blocklisted.
- Continuously monitor and adapt: Regularly track crucial deliverability metrics during the warm-up, including bounce rates, open rates, and spam complaints. Be prepared to adjust sending volumes and strategies based on these insights to maintain a healthy sending reputation and prevent being added to a blacklist.
- Ensure consistent sending patterns: During the warm-up and ongoing sending, maintain a steady and predictable sending rhythm. Avoid erratic sending patterns or sudden, large volume spikes, as these can negatively impact your subdomain's reputation and trigger spam filters and anti-phishing warnings.
Marketer view
Marketer from Email Geeks clarifies that Microsoft's anti-phishing header (which can warn about impersonation or similar-looking domains) typically targets 'cousin domains' rather than subdomains, advising a focus on proper domain usage and authentication first. He explains that 'relaxed alignment' in DKIM means the apex domain is the same, allowing an apex domain to align with its subdomain, and subdomains to align with each other. Switching to relaxed alignment is often recommended, as 'strict alignment' is rarely ideal, especially when using third-party senders because they require a dedicated '821From' (return path). The common practice for achieving alignment is to use a subdomain for the '821From' and enable relaxed alignment. Regarding warming up a subdomain, it largely depends on existing authentication: if there's already a healthy, DKIM-signed mail stream and the 'd=' (DKIM domain) is maintained for the new stream, additional warm-up might not be necessary.
29 Jan 2024 - Email Geeks
Marketer view
Email marketer from Reddit shares that warming up subdomains involves gradually increasing sending volume over time, starting with small batches to highly engaged recipients. This process helps establish a positive sending reputation with internet service providers (ISPs), signals legitimate sending behavior, and is critical for improving deliverability, preventing emails from being flagged as spam, and avoiding anti-phishing warnings associated with new or high-volume sending domains.
17 Aug 2023 - Reddit r/emailmarketing
What the experts say
3 expert opinions
Achieving strong email deliverability and preventing anti-phishing warnings hinges on two critical pillars: comprehensive email authentication and a strategic subdomain warm-up. Implementing SPF, DKIM, and DMARC across your domains, including subdomains, is fundamental. These protocols verify sender identity, combat spoofing, and guide receiving servers on how to handle unauthenticated messages, significantly bolstering trust. Furthermore, new sending subdomains must undergo a gradual volume increase, commonly known as a warm-up, to establish a positive reputation with mailbox providers. Failing to properly warm up a subdomain can lead to it being flagged by anti-spam filters or added to a blacklist, impairing inbox placement.
Key opinions
- Authentication as a core defense: SPF, DKIM, and DMARC are the bedrock of email authentication, crucial for sender verification, preventing spoofing, and providing a fundamental defense against phishing attempts across domains and subdomains.
- Dmarc's policy enforcement: DMARC is vital for deliverability and anti-phishing, as it explicitly instructs receiving mail servers on the correct handling of emails that fail SPF or DKIM validation checks.
- The necessity of subdomain warm-up: New sending subdomains require a gradual warm-up process, incrementally increasing email volume over time, which allows mailbox providers to build a trusted reputation profile for the sender.
- Consequences of inadequate warm-up: Without proper warm-up, high email volumes from new or low-reputation subdomains are highly likely to activate anti-spam filters and phishing warnings, resulting in poor inbox placement or even blocklisting.
- Alignment of 'from' headers: Optimal email authentication involves aligning the '5322.From' header (the visible sender) with the DKIM signature, alongside a separate subdomain for the '5321.From' (envelope from, used for SPF and bounce handling).
- Saas platform limitations: Some SaaS platforms, despite offering custom DKIM keys and SPF records, might hard-code their own sending domains and headers, potentially complicating efforts to achieve full domain alignment.
Key considerations
- Configure core authentication protocols: Ensure SPF, DKIM, and DMARC are correctly set up for your main domain and all sending subdomains to verify sender identity and prevent spoofing.
- Align 'from' domains for authentication: Structure your email configuration so that the '5322.From' (display sender) aligns with your DKIM signature, and use a dedicated subdomain for the '5321.From' (return-path) to optimize SPF validation.
- Execute a volume-controlled warm-up: For any new subdomain intended for email sending, implement a disciplined warm-up schedule involving a gradual increase in email volume to establish a positive sender reputation with mailbox providers.
- Understand '5321' and '5322' distinction: Clearly differentiate between the '5321.From' (envelope from address for transmission and bounces) and the '5322.From' (the 'From' header seen by recipients) when configuring your email systems.
- Anticipate receiving server modifications: Be aware that receiving mail servers, such as Microsoft's, can modify messages by adding anti-phishing headers based on their internal assessments, which is largely outside the sender's direct control.
- Verify saas platform behavior: When using third-party email platforms, confirm that they allow full control over your sending domains and do not hard-code their own, which could impact your authentication and alignment strategies.
Expert view
Expert from Email Geeks explains that you can DKIM sign with any domain, including subdomains, or the main domain using a different selector. For optimal alignment, most people use subdomains that match the '5322.From' header and DKIM signature, along with a subdomain for the '5321.From' (SPF and bounce handling). She also notes that relaxing alignment for DKIM and SPF domains is possible. Regarding Microsoft's anti-phishing headers, receiving servers can modify messages as needed, so there is little to do on the sender's side about these additions. Laura clarifies that '5321' refers to the message transmission, including the envelope from address, while '5322' refers to some headers and the message body. She has observed that some SaaS platforms, despite providing options for custom DKIM keys and SPF records, may hard-code their own sending domains and headers.
23 Dec 2022 - Email Geeks
Expert view
Expert from Spam Resource explains that configuring SPF, DKIM, and DMARC is fundamental for email authentication, which significantly improves deliverability and helps avoid anti-phishing warnings. These protocols allow receiving mail servers to verify the sender's identity, preventing spoofing and ensuring that emails appear legitimate. Specifically, DMARC (Domain-based Message Authentication, Reporting & Conformance) is crucial as it instructs receivers on how to handle emails that fail SPF or DKIM checks, providing an essential layer of protection against phishing attempts across a domain and its subdomains.
29 Jan 2025 - Spam Resource
What the documentation says
7 technical articles
Achieving robust email deliverability and proactively avoiding anti-phishing warnings requires diligent configuration of email authentication protocols for both your primary domain and any sending subdomains. Implementing SPF, DKIM, and DMARC is paramount, as these technologies collectively verify sender identity, prevent email spoofing, and provide explicit instructions to receiving mail servers on how to process unauthenticated messages. Crucially, each subdomain used for sending email necessitates its own dedicated SPF and DKIM DNS records. Furthermore, for DMARC to effectively pass and ensure optimal inbox placement, the 'From' header domain must consistently align with the domain authenticated by SPF or DKIM. While the documentation emphasizes authentication, establishing a positive sending reputation through a careful warm-up process for new subdomains remains a vital, complementary step in preventing them from being added to a blocklist or flagged for suspicious activity.
Key findings
- Foundational authentication: SPF, DKIM, and DMARC are crucial for verifying sender identity, preventing spoofing, and significantly reducing phishing attempts by instructing receiving servers on unauthenticated email handling.
- Subdomain-specific dns requirements: Each subdomain used for sending email must have its own distinct SPF (TXT record) and DKIM (CNAME or TXT record) configured to validate its origin and prevent spoofing.
- Dmarc policy as anti-phishing defense: DMARC, particularly with a 'p=quarantine' or 'p=reject' policy, acts as a primary defense, telling receiving servers to treat unauthenticated emails as suspicious and reducing anti-phishing warnings.
- Alignment for dmarc pass: For DMARC to pass, the domain in the 'From' header must align with the SPF or DKIM authenticated domain, a critical factor for preventing anti-phishing warnings, which applies equally to subdomains.
- Technical record configuration: SPF requires TXT records specifying authorized IPs (for example, 'v=spf1 include:_spf.sparkpostmail.com ~all'), while DKIM often involves CNAME records pointing to a public key provided by the email service.
Key considerations
- Configure distinct dns records for each subdomain: Ensure that separate TXT records for SPF (listing authorized sending IP addresses) and CNAME or TXT records for DKIM (pointing to the public key) are created for every subdomain used for email sending.
- Implement a robust dmarc policy: Set your DMARC policy to 'p=quarantine' or 'p=reject' to explicitly instruct receiving mail servers on how to handle emails that fail authentication, thereby significantly reducing phishing risks and improving deliverability.
- Ensure 'from' domain and authentication alignment: Critically, the domain in the 'From' header (user-visible) must align with the domain authenticated by SPF or DKIM for DMARC to pass. This is essential for both your main domain and subdomains to prevent anti-phishing warnings.
- Understand specific record types and values: Be familiar with the technical formats for SPF (for example, 'v=spf1 include:_spf.example.com ~all' TXT records) and DKIM (often CNAME records pointing to keys) to ensure correct configuration.
- Complement authentication with strategic warm-up: While authentication is key, remember that new subdomains still benefit from a gradual warm-up process to build a positive sending reputation with mailbox providers, preventing immediate blocklisting or anti-phishing flags.
Technical article
Documentation from Google Workspace Admin Help explains that configuring SPF, DKIM, and DMARC records for your domain and any subdomains used for email sending is crucial for verifying sender identity and preventing spoofing. SPF specifies authorized sending servers, DKIM adds a digital signature, and DMARC dictates how receiving servers handle unauthenticated emails, significantly reducing phishing attempts and improving deliverability.
5 Jun 2023 - Google Workspace Admin Help
Technical article
Documentation from Mailgun explains that for each subdomain used for sending email, you must configure separate DNS records for SPF and DKIM. While SPF often requires listing authorized sending IPs, DKIM involves adding a CNAME record to your DNS for the specific subdomain. Implementing DMARC at the organizational domain level generally covers subdomains, but proper alignment of SPF and DKIM with the 'From' domain is essential for DMARC pass, which helps prevent anti-phishing warnings.
17 Jun 2025 - Mailgun Documentation
Related resources
7 resources
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