What resources help explain IP addresses, sub-domains and domains reputation for sending?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 May 2025
Updated 18 Aug 2025
7 min read
Navigating the complexities of email deliverability can feel like a maze, especially when it comes to understanding how IP addresses, subdomains, and domains contribute to your sending reputation. It's a critical area, as a healthy reputation directly impacts whether your emails land in the inbox or the spam folder. Building and maintaining this reputation requires a deep dive into technical configurations and sending practices.
Many factors influence how mailbox providers (like Google and Outlook) perceive your emails. This includes not just the content of your messages, but also the digital identity behind them: your IP address and domain. Missteps in managing these can lead to emails being blocked or flagged as spam, severely hindering your communication efforts. The good news is that there are many resources available to help you grasp these concepts and apply best practices.
I'll guide you through key concepts, authoritative resources, and practical tips to help you understand and maintain a strong email sending reputation across your IP addresses, subdomains, and primary domains.
The foundation: IP address reputation
Your IP (Internet Protocol) address is essentially the street address of your email server. When you send an email, the recipient's server notes the IP address from which it originated. This address builds a reputation based on the sending behavior associated with it. If emails sent from an IP address consistently generate spam complaints, bounce, or hit spam traps, that IP's reputation will suffer, leading to lower inbox placement rates.
There are two main types of IP addresses for email sending: dedicated IPs and shared IPs. A dedicated IP means only your emails originate from that address, giving you full control over its reputation. Shared IPs, on the other hand, are used by multiple senders. While they can be a good starting point for low-volume senders, your reputation can be affected by the sending practices of others using the same IP.
Understanding sending IPs and domains is crucial. To maintain a healthy IP reputation, consistent sending volume and good list hygiene are paramount. ISPs monitor metrics like complaint rates, bounce rates, and spam trap hits. High rates in any of these areas signal poor sending practices, which can quickly lead to your IP being blocklisted (or blacklisted). I always recommend consulting best practices from industry groups, such as M3AAWG's Sender Best Common Practices, for comprehensive guidance.
Warming up a new IP address is also vital. This involves gradually increasing your sending volume over time to build a positive sending history with mailbox providers. Skipping this step can trigger spam filters, as sudden bursts of email from a new or dormant IP often resemble spam.
Understanding domain reputation
While IP reputation focuses on the sender's infrastructure, domain reputation centers on the identity of the sender, specifically your domain name. This is often considered even more critical than IP reputation because it ties directly to your brand. Mailbox providers assess your domain's reputation based on past sending behavior, recipient engagement, and compliance with email authentication protocols.
Domain reputation is highly influenced by email authentication standards like SPF, DKIM, and DMARC. Proper configuration of these records tells recipient servers that your emails are legitimate and haven't been tampered with. Without them, even well-intentioned emails may be flagged as suspicious, regardless of your IP reputation.
A good resource to start with is Spamhaus's guide to domain reputation. It explains what it is and how it impacts your email program. The overall health of your domain reputation is critical for email deliverability. If your domain has a poor reputation, it can affect all emails sent from it, regardless of the IP address used.
IP reputation
Focus: Measures the trustworthiness of the specific IP address from which emails originate.
Management: Requires careful IP warming and consistent sending behavior.
The strategic use of subdomains
Subdomains are like branches of your main domain tree. For example, if your main domain is example.com, you might use marketing.example.com for marketing emails and transactional.example.com for transactional messages. The primary reason for using subdomains for email sending is to isolate reputation. If one type of email (e.g., promotional) experiences deliverability issues, it won't necessarily damage the reputation of your main domain or other sending subdomains.
This isolation is a significant advantage. Mailbox providers often treat subdomains with their own unique sending reputation, even if they share the same root domain. This means that even if your automated email subdomain gets blocklisted, your primary domain, used for corporate communications, remains unaffected. However, there's also an influence; a severely damaged parent domain reputation can still negatively affect its subdomains to some extent.
When considering using subdomains, ensure each is properly authenticated with its own SPF, DKIM, and DMARC records. This step is crucial for establishing trust with mailbox providers and ensuring deliverability. You can learn more about how parent domain reputation impacts subdomains for a deeper understanding.
It's also important to remember that each subdomain requires its own warm-up period, similar to a new IP address, to build a positive sending history before reaching full volume.
Monitoring and improving your sending reputation
Monitoring your IP and domain reputation is an ongoing process. Tools like Google Postmaster Tools offer invaluable insights into how Google perceives your sending reputation, including your IP and domain. Utilizing these tools can help you proactively identify and address potential issues before they escalate.
Beyond monitoring, consistent effort is required to improve and maintain a healthy reputation. This includes diligent list hygiene, regularly removing inactive or invalid email addresses to reduce bounces and spam trap hits. Engaging content that encourages opens and clicks also positively influences your reputation, as it signals to mailbox providers that your emails are valued by recipients. Regularly auditing your email program, as outlined in guides on improving email IP and domain reputation, is a continuous best practice.
If your IP or domain finds itself on an email blacklist (or blocklist), swift action is necessary. Being listed means your emails will likely be rejected by many recipients. Resources detailing what happens when your domain is blacklisted can provide immediate guidance. The process of delisting often involves identifying the root cause of the listing, remediating the issue, and then requesting removal from the specific blocklist. Remember that prevention through good sending practices is always better than cure.
Views from the trenches
Best practices
Actively monitor your bounce rates, complaint rates, and spam trap hits to catch issues early.
Segment your email lists and use separate subdomains for different types of mail, like transactional and marketing emails.
Implement and correctly configure SPF, DKIM, and DMARC for all your sending domains and subdomains.
Regularly clean your email lists to remove inactive or invalid addresses, minimizing hard bounces.
Warm up new IP addresses and subdomains gradually by sending small volumes initially and slowly increasing over time.
Common pitfalls
Ignoring bounce and complaint reports, which are clear signals of reputation issues.
Sending high volumes of email from a new or un-warmed IP address or subdomain.
Failing to set up or incorrectly configuring email authentication records (SPF, DKIM, DMARC).
Using a single domain or IP address for all email sending, increasing risk of widespread impact.
Sending to old, unengaged, or purchased lists, which can lead to spam trap hits and blocklistings.
Expert tips
Leverage Google Postmaster Tools for invaluable insights into your deliverability performance with Google.
Consistently send engaging content that encourages positive recipient interaction, which boosts your reputation.
Consider dedicated IPs if your sending volume is high and consistent enough to justify the control and effort.
Understand that some blocklists (blacklists) are private, so you may not always know if you're listed.
Maintain a clear separation between marketing and transactional emails to protect critical communications.
Expert view
Expert from Email Geeks says M3AAWG's Sender Best Common Practices is an excellent starting point for anyone looking to understand email deliverability deeply.
2022-06-01 - Email Geeks
Expert view
Expert from Email Geeks says there are also many other best practices documents published by M3AAWG that are valuable for the industry.
2022-06-01 - Email Geeks
Maintaining a healthy sending identity
Building and maintaining a strong reputation for your IP addresses, subdomains, and domains is fundamental to successful email deliverability. It's a multifaceted process that involves understanding technical standards, monitoring sending performance, and adhering to best practices consistently.
By leveraging authoritative resources, implementing proper email authentication, strategically using subdomains, and continually monitoring your sending metrics, you can ensure your emails consistently reach the inbox. A healthy sending reputation not only improves deliverability but also strengthens your brand's trust and credibility with your audience.
Google and 
Outlook) perceive your emails. This includes not just the content of your messages, but also the digital identity behind them: your IP address and domain. Missteps in managing these can lead to emails being blocked or flagged as spam, severely hindering your communication efforts. The good news is that there are many resources available to help you grasp these concepts and apply best practices.
