What ISP information is most useful for email deliverability?

Key findings

• Postmaster pages: Official ISP postmaster pages are a primary source for guidelines, tools, and support for deliverability issues.
• Authentication standards: DMARC, SPF, and DKIM authentication are fundamental requirements, with ISPs increasingly enforcing these standards rigorously. Proper configuration is vital, as outlined in our guide on DMARC, SPF, and DKIM.
• ISP-specific quirks: Unique behaviors or policies of individual ISPs (e.g., how they handle DMARC failures, support response times) are highly valuable insights.
• Domain reputation: ISPs are increasingly relying on domain reputation, not just IP reputation, as a key factor in filtering decisions, especially those supporting IPv6 MX records. Learn more about how ISPs track engagement.
• Feedback loops (FBLs): While some aspects of FBLs may be evolving, they still offer useful data on spam complaints for some senders.

Key considerations

• Blacklist impact: Understanding which specific blocklists (or blacklists) have the most impact on deliverability to a given ISP is important for rapid issue resolution.
• Third-party filtering: Being aware of any third-party filtering technologies an ISP relies on (e.g., Cloudmark) can help in diagnosing delivery problems.
• Bounce codes: Knowing ISP-specific or unusual bounce codes helps interpret delivery failures accurately, differentiating between temporary issues and hard blocks. For a comprehensive resource, refer to the SMTP Field Manual.
• Connection limits: While MTAs often handle these automatically, understanding ISP-specific connection limits can be vital for senders using less conventional systems or seeking granular control.
• BIMI adoption: Tracking an ISP's adoption or plans for Brand Indicators for Message Identification (BIMI) can inform future email branding strategies.

Key opinions

• FBL utility: Many marketers still find FeedBack Loops (FBLs) valuable for understanding spam complaints and maintaining list hygiene.
• Postmaster page importance: Direct links to ISP postmaster pages are considered highly helpful, particularly when needing to submit support tickets for deliverability issues, as discussed in our guide on contacting mailbox providers.
• ISP quirks: Information on specific ISP behaviors, such as Gmail's silent actions on support tickets or Outlook.com's DMARC compliance rules, is critical for daily operations.
• Bounce code clarity: Clear definitions of obscure or ambiguous bounce codes from specific ISPs can help marketers quickly identify and resolve delivery blocks.
• Domain reputation recovery: Strategies like 'low and slow' sending to engaged users are often cited as optimal for improving domain reputation with major ISPs like Gmail. For more details on recovery, see our article on recovering domain reputation.

Key considerations

• Authentication variations: Marketers need to know if an ISP (like Microsoft/Hotmail) still checks for older authentication methods such as SPF2.0 or SenderID.
• Blocklist influence: Understanding which blacklists (or blocklists) are most influential for specific ISPs helps prioritize de-listing efforts.
• Deliverability transparency: Marketers often desire more transparency from ISPs regarding their filtering processes and the specific factors affecting deliverability, as highlighted by EmailTooltester.com's best practices.
• ESP interaction: For those not owning sending IPs, guidance on dealing with deliverability issues, especially with major ISPs like Outlook.com, is highly valued.

Key opinions

• Outdated practices: Experts largely agree that whitelisting is no longer a relevant factor for deliverability, and the value of FBLs has diminished, though they still provide some insight into complaint rates.
• DMARC and authentication: There's strong consensus on the critical importance of DMARC and other authentication requirements. ISPs actively verify these to assess sender legitimacy, as detailed in our guide on fixing common DMARC issues.
• MTA handling: While Message Transfer Agents (MTAs) generally automate backoffs and connection limits, some experts suggest that the ability to control these settings (for specialized or smaller senders) can still be valuable.
• Domain reputation dominance: ISPs that utilize IPv6 MX records are likely already heavily relying on domain reputation, indicating a broader industry shift towards this metric over mere IP reputation.
• Current ISP policies: Keeping up with dynamic ISP policies, including their DMARC settings for their own domains and their adherence to senders' DMARC policies, is seen as crucial.

Key considerations

• Data availability: A challenge is that ISPs often cease publishing detailed technical information like connection limits, making it harder for senders to access this data directly.
• Bounce code resources: Leveraging community-maintained resources, such as the SMTP Field Manual, for interpreting specific bounce messages from various ISPs is highly recommended.
• Sending IP management: For those managing their own sending infrastructure, understanding best practices for setting email domain connection limits to major ISPs can optimize performance and deliverability.
• Authentication myths: It's important to debunk myths, such as whether Microsoft/Hotmail still explicitly checks for SPF2.0 or SenderID, to focus on the most effective authentication methods.
• Proactive monitoring:Experts at WordtotheWise.com advise on continuously monitoring ISP responses and engagement metrics to maintain optimal deliverability.

Key findings

• Postmaster sites: ISPs like Gmail and Outlook provide specific postmaster websites (e.g., Google Postmaster Tools) that offer insights into deliverability metrics and guidelines. Learn more about Google Postmaster Tools V2.
• Authentication standards: RFCs (Requests for Comments) define standards like SPF (RFC 7208), DKIM (RFC 6376), and DMARC (RFC 7489), which are crucial for verifying sender identity and preventing spam and phishing.
• Sender reputation: Official documentation consistently highlights sender reputation (both IP and domain) as a primary factor influencing filtering decisions. This includes engagement metrics, bounce rates, and complaint rates. Our guide on technical solutions for deliverability provides further detail.
• Bounce messages: SMTP response codes (e.g., 5xx series for permanent failures, 4xx for temporary failures) provide standardized information about why an email could not be delivered.

Key considerations

• Compliance updates: Mailbox providers, such as Outlook.com, regularly update their sender requirements, necessitating continuous monitoring and adaptation to ensure compliance. See our guide on complying with Outlook's new requirements.
• Explicit consent: Documentation consistently emphasizes the importance of obtaining explicit consent for sending emails (opt-in) to maintain a healthy sender reputation and avoid spam complaints.
• Unsubscribe mechanisms: Providing clear and easy-to-use unsubscribe options, often through List-Unsubscribe headers, is a documented best practice to minimize complaints and improve deliverability.
• Content guidelines: While less technical, documentation often includes guidelines on email content to avoid spam filters, such as avoiding excessive capitalization, spammy phrases, or broken links.
• API access: Documentation for tools like Microsoft's SNDS (Smart Network Data Services) API highlights the programmatic access available to senders for monitoring their sending health and reputation.