What ISP information is most useful for email deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 17 Jul 2025
Updated 19 Aug 2025
6 min read
Email deliverability often feels like navigating a complex maze, and at the heart of it are Internet Service Providers (ISPs). These gatekeepers decide whether your emails land in the inbox, the spam folder, or are rejected outright. To ensure your messages consistently reach their intended audience, it's crucial to understand what information ISPs consider most important when evaluating incoming mail.
The signals ISPs use to filter email are constantly evolving. What was once a key factor years ago, like general whitelisting practices, is largely outdated, replaced by more sophisticated metrics. Focusing on the right data points and understanding ISP behaviors can significantly impact the success of your email campaigns.
My goal is to shed light on the most valuable ISP-related information you should prioritize to improve your email deliverability. This includes everything from how your reputation is built to the specific technical requirements they enforce, and even their unique operational quirks.
Sender reputation and trust signals
At the core of an ISP's decision-making process lies your sender reputation. This isn't a single numerical score, but a comprehensive assessment based on various factors associated with your IP address and domain. A robust, positive reputation signals to mailbox providers (MBPs) that you are a legitimate and trustworthy sender.
ISPs meticulously track metrics such as your spam complaint rate, bounce rate, and user engagement, which includes opens, clicks, and replies. High complaint rates or frequent bounces indicate poor list hygiene or unwanted content, quickly deteriorating your standing. Conversely, consistent strong engagement signals that your recipients value and interact with your messages, positively influencing your reputation.
Your IP reputation, directly tied to the specific IP address used for sending, and your domain reputation, linked to your domain name, are both critically important. While a shared IP can be influenced by other senders, dedicated IPs require careful and consistent IP warming strategies and positive sending practices to build trust with ISPs and maintain a strong standing.
IP reputation
Tied to sending IP address.
Influenced by volume, frequency, and bounce rates from that IP.
New IPs require warming to establish trust.
Can be impacted by other senders on shared IPs.
Domain reputation
Tied to sending domain (e.g., yourdomain.com).
Influenced by engagement, spam complaints, and authentication.
Crucial for long-term deliverability, even with IP changes.
Many ISPs, especially those with IPv6 MX records, focus heavily on domain reputation.
Authentication and policy enforcement
Email authentication protocols like SPF, DKIM, and DMARC are foundational pieces of information that ISPs use to verify sender legitimacy. These DNS records help prevent spoofing and phishing by assuring the receiving server that an email truly originated from your domain.
New Gmail and Yahoo requirements
Google and Yahoo have recently introduced stricter requirements for bulk senders. For high-volume sending, it is now mandatory to implement DMARC, SPF, and DKIM, keep spam complaint rates low, and ensure easy unsubscription methods. Failure to comply can lead to email rejection or direct placement in spam folders.
Many major ISPs, including those mentioned, have significantly tightened their requirements for email authentication. Understanding their specific compliance expectations is vital to avoid messages being rejected or routed to spam folders. Regularly checking Google Postmaster Tools can provide valuable insights into your authentication performance.
Furthermore, understanding an ISP's own DMARC policy for its domains (e.g., Yahoo's reject policy) and how they honor the sending domain's policy offers valuable insight into their overall security posture and filtering priorities. Proper DMARC configuration is a strong signal of legitimate email practices.
Beyond universal best practices, each ISP often has its own unique quirks or specific behaviors that can impact deliverability. For example, a particular ISP might apply different filtering rules if a recipient has your address in their contact list, or handle DMARC-failing emails with varying levels of strictness.
The response times and methods for support tickets also vary significantly across providers. Some, like Gmail, might not reply directly but will take action, while others, like Outlook.com (formerly Oath), might have longer response times. Knowing these operational differences can help you manage expectations when dealing with deliverability issues.
Feedback loops (FBLs) remain a valuable, though evolving, source of information. These reports notify senders when their emails are marked as spam by recipients, providing critical data to identify and remove unengaged or complaining users from your lists. This helps in maintaining a healthy sender reputation and tracking email engagement.
ISP
Support response
DMARC handling quirks
Google
Often no direct reply, but action taken.
Strict DMARC enforcement for bulk senders.
Yahoo
Replies can take up to a week.
Strong DMARC enforcement; rejects mail not passing policy.
Outlook.com
Varies; often requires use of sender support forms.
May not reject DMARC-failing emails if address in contact book.
Technical signals and ongoing monitoring
ISPs also utilize technical signals embedded within email headers and server responses. Bounce codes, for instance, provide specific reasons for email rejections, helping diagnose issues ranging from temporary deferrals to permanent blocks. Analyzing these can offer direct clues about why your emails aren't reaching the inbox. For a comprehensive reference on SMTP responses, the SMTP Field Manual is quite helpful.
While many Message Transfer Agents (MTAs) automatically handle connection limits and back-offs, understanding these parameters, especially for smaller senders or those using less conventional systems, can be crucial. Some platforms provide granular control over connection settings, which can be optimized for specific ISPs to improve throughput and avoid temporary blocks.
Constant vigilance over your IP and domain status on various email blocklists (also known as blacklists) is non-negotiable. ISPs frequently consult these lists, and an entry can severely impact your deliverability, leading to widespread rejections. Being aware of which specific blacklists an ISP predominantly relies on can help prioritize your monitoring efforts. Building email deliverability best practices is an ongoing process.
Navigating the deliverability landscape
In the intricate world of email deliverability, the information gleaned from ISPs is indispensable. From understanding how they gauge your sender reputation through engagement and complaint rates to deciphering their specific authentication requirements, every piece of data contributes to a clearer picture.
Ultimately, success in email deliverability boils down to trust. Providing ISPs with the right signals—through proper authentication, good sending practices, and a clean list—will ensure your messages consistently reach your audience, fostering positive engagement and stronger email programs.
Views from the trenches
Best practices
Proactively monitor your sender reputation using tools like Google Postmaster Tools.
Segment your lists to send to engaged users, improving open and click rates.
Implement DMARC with a strong policy to protect your brand and improve deliverability.
Warm up new IP addresses or domains gradually to build trust with ISPs.
Common pitfalls
Ignoring bounce codes, which indicate underlying deliverability issues.
Not using SPF, DKIM, and DMARC, leading to authentication failures.
Sending emails to unengaged lists, resulting in high spam complaints.
Failing to monitor blacklists, missing crucial signs of reputation damage.
Expert tips
Pay close attention to ISP-specific bounce codes; they often provide direct, actionable insights into filtering issues.
Leverage Feedback Loops (FBLs) as they offer critical data on spam complaints, even if their mechanisms evolve.
Prioritize maintaining a clean, engaged email list to positively influence both IP and domain reputation.
Understand that while some MTA functions are automated, knowing connection limits can still be beneficial for custom setups.
Expert view
Expert from Email Geeks says: Feedback loops are still valuable for obtaining insights into spam complaint reports, and API tools access information, such as SNDS API, can also be beneficial for monitoring.
2019-12-17 - Email Geeks
Marketer view
Marketer from Email Geeks says: Postmaster pages are extremely helpful, especially when you need to submit tickets for deliverability issues.
Google and 
Yahoo have recently introduced stricter requirements for bulk senders. For high-volume sending, it is now mandatory to implement DMARC, SPF, and DKIM, keep spam complaint rates low, and ensure easy unsubscription methods. Failure to comply can lead to email rejection or direct placement in spam folders.
Outlook.com (formerly Oath), might have longer response times. Knowing these operational differences can help you manage expectations when dealing with deliverability issues.
