How does Google's reputation system work and what are the best practices for warming up IPs?
Matthew Whittaker
Co-founder & CTO, Suped
Published 14 Jun 2025
Updated 16 Aug 2025
7 min read
For anyone involved in email marketing or transactional email, ensuring your messages land in the inbox is a top priority. A critical component of achieving this goal, especially when sending to Google Workspace users, hinges on understanding sender reputation. Google, as one of the largest inbox providers, employs a sophisticated system to evaluate the trustworthiness of incoming mail, affecting whether your emails reach the primary inbox, promotions tab, or the dreaded spam folder.
A key practice in building and maintaining this trust is IP warming. Whether you are setting up a new email sending infrastructure or re-engaging with an old one, a structured IP warming strategy is essential. I often see senders struggle with deliverability, only to find they skipped or rushed this crucial step.
How Google's reputation system works
Google's reputation system is dynamic and multi-faceted, leveraging machine learning to assess the legitimacy and quality of emails. It doesn't rely on a single factor but rather on a combination of signals to determine sender reputation. This includes the reputation of your sending IP address, your domain (the domain in your sender address), and the engagement of recipients with your emails.
What is particularly important is the interplay between domain and IP reputation. If your domain has a strong reputation, it can positively influence the reputation of a new or unknown IP address. Conversely, if your domain's reputation suffers, even a good IP can be affected. For instance, if messages are marked as spam, every associated feature, including domain and IP reputation, takes a hit. This means your domain's reputation (validated by authentication like SPF and DKIM) is crucial.
Google also heavily weighs user engagement. High spam complaint rates, low open rates, and frequent deletions without opening signal to Google that your emails are not valuable to recipients. This negative feedback can significantly degrade your overall sender reputation. You can monitor many of these metrics using Google Postmaster Tools, which provides valuable insights into your IP and domain reputation specifically as Google sees it.
The importance of IP reputation and warming
IP reputation is essentially a trust score assigned to your sending IP address by Internet Service Providers (ISPs), including Google. A high IP reputation indicates that you are a legitimate sender, while a low score suggests potential spamming activity. New IP addresses, or those that have been dormant, start with a neutral or unknown reputation. To build this trust, you must embark on a process known as IP warming.
IP warming involves gradually increasing the volume of emails sent from a new or cold IP address over a period of time. This slow and steady approach allows ISPs to observe your sending behavior and verify that you are a consistent, responsible sender. Without proper warming, a sudden surge in email volume from an unknown IP can trigger spam filters, leading to immediate blocklists (or blacklists) and poor inbox placement.
While shared IP addresses may not always require extensive warming (as the reputation is shared with other senders), dedicated IPs absolutely do. Dedicated IPs give you full control over your sending reputation, making the warming process even more critical to establish your own positive standing with ISPs.
Best practices for IP warming
Implementing a robust IP warming strategy requires careful planning and execution. Here are some best practices I recommend for new or dormant IPs:
Start with your most engaged recipients. When you begin warming, it's crucial to send emails to your most active and engaged subscribers first. These are recipients who consistently open, click, and reply to your emails, demonstrating positive engagement signals to ISPs. This positive interaction helps build initial trust and tells Google's filters that your mail stream is wanted.
Gradual volume increase: Begin with a low sending volume (e.g., a few thousand emails per day) and slowly increase it over several weeks. A common approach is to double your sending volume weekly, but always be prepared to slow down if you encounter any deliverability issues. This gives Google's machine learning filters time to learn and adapt to your sending patterns.
Consistent sending cadence: Try to maintain a consistent sending frequency and volume during the warming period. Erratic sending patterns can raise red flags with ISPs. For example, if you send 10,000 emails one day and then 100,000 the next, it can negatively impact your reputation.
Monitor your reputation: Regularly check your IP and domain reputation using tools like Google Postmaster Tools. Pay close attention to spam rates, IP reputation, and domain reputation. If you see any drops, it's a sign to slow down your sending volume and investigate the cause.
Maintain list hygiene: Ensure your email list is clean and free of invalid or inactive addresses. High bounce rates and hitting spam traps can severely damage your reputation during warming. Regular list cleaning is crucial for long-term deliverability. You should also be prepared to reduce sending if your bounce rates or unsubscribe rates are high.
The duration of an IP warming period can vary, typically ranging from 4 to 8 weeks, depending on your sending volume and recipient engagement. Patience is key, as rushing the process can lead to long-term deliverability issues.
Managing challenges during IP warming
Even with the best planning, you might encounter challenges during IP warming. It's not uncommon to see temporary dips in your IP or domain reputation, or even land on an email blocklist (also called a blacklist) as ISPs assess your new sending patterns. The most important thing is to react quickly and appropriately. For example, some common blocklists include Spamhaus and Spamcop, among others.
If you notice a sudden drop in deliverability or a spike in spam complaints, immediately reduce your sending volume. Investigate the cause using your email logs and Postmaster Tools. This could be due to a segment of your audience being less engaged, or perhaps a spam trap. Use our blocklist checker to see if your IP or domain has been listed, and then work to request delisting if necessary. You can also use blocklist monitoring tools to stay ahead of these issues.
Remember, IP warming is an ongoing process of building trust. It's about demonstrating consistent, wanted sending behavior to Google's filters over time. The filters are continually learning, and your practices should adapt accordingly. Ensuring your email authentication (SPF, DKIM, DMARC) is correctly configured is also paramount, as it provides verifiable identity to your emails, further contributing to trust.
Building a strong sender reputation
Building a strong sender reputation with Google, and other ISPs, is a continuous journey that requires patience, strategic planning, and diligent monitoring. By understanding how Google’s reputation system works and implementing best practices for IP warming, you can significantly improve your email deliverability, ensuring your messages reach their intended recipients.
Views from the trenches
Best practices
Start warming new IPs or domains with your most engaged subscribers to build initial trust.
Gradually increase sending volume, being careful not to spike suddenly, which can trigger spam filters.
Maintain consistent sending patterns and frequencies during the warming period.
Regularly monitor your IP and domain reputation using Google Postmaster Tools.
Ensure all email authentication (SPF, DKIM, DMARC) records are correctly configured and aligned.
Common pitfalls
Rushing the IP warming process by sending too many emails too quickly, leading to blocklists (blacklists).
Failing to segment your audience and sending to disengaged users early in the warming process.
Ignoring negative feedback signals, like high spam complaints or bounce rates, and not adjusting volume.
Not regularly checking blocklists (blacklist) and failing to request delisting when necessary.
Inconsistent sending schedules that make your traffic appear suspicious to ISPs.
Expert tips
The balance between IP and domain reputation is critical; a strong domain can help a new IP, but a bad domain will harm any IP.
Google's machine learning filters are highly sophisticated and continuously learn from sending patterns and recipient engagement.
It's about introducing yourself to the filters and allowing them to adapt to your new email stream.
Consider a warming schedule that doubles your volume weekly after an initial slow period (3-4 days).
Be aware that different mailbox providers (MBPs) may have varying sensitivities to warming processes.
Expert view
Expert from Email Geeks says that if a message is marked as spam, every associated feature, including domain and IP reputation, takes a hit. If the domain's reputation (SPF or DKIM) is sufficiently impacted, emails will go to spam even if the IP reputation is not inherently bad, and that IP's reputation will eventually suffer as well.
2022-03-29 - Email Geeks
Expert view
Expert from Email Geeks says that when adding new IPs, if the authenticated domain is well-known and trusted, the new IP can start to inherit a good reputation from it, despite having an unknown reputation initially.
