When purchasing IP blocks, how long should they be unused and what checks are needed?
Michael Ko
Co-founder & CEO, Suped
Published 8 Jun 2025
Updated 19 Aug 2025
7 min read
Acquiring new IP blocks for your email sending infrastructure is a significant step, especially as IPv4 addresses become increasingly scarce. It is not as simple as purchasing a block and immediately deploying it. The history and past usage of these IPs can profoundly impact your email deliverability from day one. Sending emails from an IP block with a poor reputation can lead to immediate blocklists (or blacklists) and low inbox placement rates, negating the very purpose of your investment.
My experience has shown that diligence before and after acquisition is paramount. You need to understand the prior life of these IP addresses and prepare them correctly for your email operations. This involves a crucial resting period and a series of comprehensive checks to safeguard your sender reputation.
The goal is to ensure that when you begin sending, your new IP block is as clean as possible, allowing for a smooth IP warming process and optimal inbox placement.
The importance of IP block reputation
The previous usage of an IP block directly influences its current reputation. If the IPs were previously used for spamming, phishing, or other abusive activities, they might already be listed on various blocklists (or blacklists). Starting to send emails from such IPs will almost certainly result in your messages being flagged as spam or outright rejected by mailbox providers.
This inherited poor reputation can be incredibly difficult to overcome. It will lead to deliverability issues, damage your sending domain's reputation, and ultimately impact your business communication. Recovering from a bad IP reputation can take significant time and effort, far more than the initial due diligence.
A clean IP history is the foundation for effective email deliverability. Without it, even the best email content and list hygiene practices will struggle to achieve optimal inbox placement. This is why understanding the importance of IP reputation is the first critical step in purchasing IP blocks.
Risks of a poor IP reputation
Deliverability issues: Emails landing in spam folders or being rejected outright.
Blocklisting: Being added to public and private blacklists (or blocklists).
Brand damage: Your brand associated with spam or malicious activity.
Costly recovery: Time and resources spent on delisting and reputation repair.
How long should IP blocks be unused?
The duration an IP block should remain unused (or fallow) before you use it for email sending is a subject with varying opinions. Some experts suggest as little as a few days or a couple of weeks, especially if the block has already been idle for some time. Others advocate for a more cautious approach, recommending up to six months of inactivity to thoroughly clear any lingering negative reputation.
The primary reason for a resting period is to allow time for the IP addresses to be removed from any blacklists (or blocklists) they might be on, and for mailbox providers to reset their internal reputation scores. These scores are dynamic and influenced by recent sending behavior. If an IP has been heavily abused, it might take longer for this negative history to dissipate naturally.
I often recommend a minimum of two weeks of inactivity, particularly if there's any uncertainty about the block's recent history. However, the most reliable method is to conduct small, controlled test sends after a short resting period. This allows you to gauge the current reputation and decide if further inactivity is necessary. For guidance on preparing new IPs, you can look into whether a 30-day waiting period is needed.
Essential checks before using a new IP block
Beyond simply checking for existing blacklists (or blocklists), several other crucial checks should be performed when acquiring an IP block to ensure its health and suitability for email sending. This due diligence can save you significant headaches down the road.
Previous owner's reputation: Research the entity that previously owned or leased the IP block. If they had a history of spamming, that negative association might carry over. You can use a WHOIS lookup to find out ownership details.
RIR registration data: Verify the IP block's registration with the relevant Regional Internet Registry (RIR) like ARIN, RIPE NCC, APNIC, LACNIC, or AFRINIC. Ensure the transfer process is legitimate and recorded correctly.
Spam trap hits: While difficult to check directly, if the IPs were heavily associated with spam traps, even a resting period might not fully clear the impact. This often reveals itself during initial warming sends. Find out more about spam traps.
An IPv4 block being offered for sale should ideally have a clear history or at least verifiable documentation regarding its past usage. I always advise asking the seller for any historical reputation reports or delisting confirmations if available. Transparency from the seller is a good sign.
Example: Performing a WHOIS lookupbash
whois 192.0.2.0
Warming up your newly acquired IP blocks
Once you've acquired your IP block, completed your checks, and allowed for a resting period, the next critical step is IP warming. This involves gradually increasing your email sending volume over time, allowing mailbox providers to assess your sending practices and build a positive reputation for your new IPs. Even a perfectly clean IP block needs to be warmed up, as it lacks established trust with ISPs.
Skipping or rushing the IP warming process is one of the quickest ways to land your new IP addresses on blacklists and have your emails sent directly to the spam folder. Mailbox providers are wary of new IPs suddenly sending high volumes, as this is a common tactic used by spammers.
The warming schedule depends on factors like the volume of mail you intend to send, the engagement levels of your recipients, and the sensitivity of the mailbox providers you're sending to. I always advise a conservative approach. For a more detailed guide, check out our resources on how long it takes to warm up an IP address.
Consistency is key during warming. Send daily, gradually increasing volume, and monitor your deliverability closely. Pay attention to bounce rates, complaint rates, and inbox placement across different providers. You can find more information on best practices for dedicated IP warm-up.
Views from the trenches
Best practices
Always perform comprehensive due diligence on IP blocks before purchase, including historical checks and blacklist status.
Implement a gradual IP warming schedule tailored to your sending volume and target audience.
Maintain consistent sending volumes during the warming period and avoid sudden spikes.
Actively monitor deliverability metrics, including bounce rates and complaint rates, to detect issues early.
Common pitfalls
Skipping the resting period, assuming newly purchased IPs are clean by default.
Rushing IP warming by sending high volumes too quickly, leading to blocks and poor reputation.
Not checking multiple blacklists or overlooking regional blocklists.
Failing to research the previous owner's sending history and associated reputation.
Expert tips
If an IP block had recent activity, allowing a two-week resting period can often be sufficient.
The surest way to gauge the health of new IPs is through controlled test sending to various mailbox providers.
Be overly cautious with IPs that have a questionable or unknown history; a longer rest may be prudent.
Beyond blacklists, assess the RIR registration data to ensure a legitimate transfer of the IP block.
Marketer view
Marketer from Email Geeks says that beginning with test sending to warm up new IPs is an effective way to gauge their reputation, especially if previous usage is unknown.
May 20, 2019 - Email Geeks
Marketer view
Marketer from Email Geeks says that for IPs with known issues, a resting period of two weeks before re-warming can be beneficial.
May 20, 2019 - Email Geeks
Purchasing IP blocks for email sending is a strategic decision that requires careful planning and execution. The success of your email program heavily relies on the reputation of your sending IPs, which is why thorough pre-purchase checks and a well-thought-out warming strategy are indispensable.
By understanding the importance of IP reputation, allowing sufficient resting time for new blocks, performing diligent checks on their history, and then diligently warming them up, you can lay a strong foundation for excellent email deliverability and avoid common pitfalls that lead to blocklistings and spam folder placements.
