Should I email unknown status users after email validation during transactional domain warming?
Michael Ko
Co-founder & CEO, Suped
Published 5 Aug 2025
Updated 19 Aug 2025
5 min read
When you're warming up a transactional domain, the goal is to gradually build sender reputation with mailbox providers. This process involves slowly increasing sending volume to show that your email traffic is legitimate and wanted. A critical step in maintaining a healthy sender reputation is list hygiene, often done through email validation services.
However, sometimes these validation services return an "unknown" status for a portion of your list, meaning they couldn't definitively classify the email address as valid or invalid, often due to no server response. The question then becomes, what do you do with these contacts, especially during a crucial domain warming phase? Sending to them might feel like a risk, but not sending means potentially missing out on legitimate recipients.
Deciphering 'unknown' status in email validation
The "unknown" status from an email validation service is a common yet frustrating outcome. It typically means the validation tool couldn't get a definitive response from the recipient's mail server. This isn't necessarily a hard bounce, but rather an indication that the server either timed out, was configured to defer responses, or used greylisting tactics.
Many email service providers queue messages for these addresses and retry delivery over hours or even days. If your ESP doesn't provide granular insight into these retries, you might see a disparity between "sent" and "delivered" numbers, without immediately knowing the final outcome. This can artificially lower your reported deliverability rate, even if a bounce isn't technically registered yet.
Some mailbox providers, like Yahoo (and former Verizon/AOL properties), are known for their more aggressive deferral strategies, often accepting recipients during the initial SMTP handshake and only rejecting after the data is transmitted. This makes it challenging for email validation services to get an immediate, conclusive status.
Impact on transactional domain warming
The primary concern with sending to unknown status users during transactional domain warming is the potential impact on your sender reputation. While a low bounce rate and zero spam complaints are positive indicators, a high percentage of undelivered emails, even if not immediately bounced, can signal to mailbox providers that you're sending to unengaged or questionable addresses.
Mailbox providers assess your sending habits closely during the warming period. Sending to addresses that frequently defer or never acknowledge receipt can lead to throttling, filtering to the spam folder, or even placement on email blacklists or blocklists. This can hinder your warming efforts and make it harder to achieve optimal inbox placement for your legitimate transactional emails in the long run. Understanding your domain reputation is crucial.
The risk of sending to 'unknowns'
While immediate bounces may be low, a high rate of unconfirmed deliveries or deferred responses for "unknown" addresses can negatively influence your sender reputation. This can lead to increased spam filtering and deliverability issues. It's especially critical during domain warming for email deliverability as you are actively building trust.
Potential for spam traps: Some unknown addresses could eventually resolve to spam traps, which are highly damaging to sender reputation.
Lower deliverability rates: Even without hard bounces, repeated deferred deliveries for a large segment can drag down your overall deliverability statistics with your ESP, indicating poor list quality.
Strategic handling of unknown users
Given the uncertainties, a cautious approach is generally recommended. During the critical early stages of transactional domain warming, it's best to prioritize sending to email addresses with a confirmed "valid" status. These are the contacts most likely to positively engage, helping you build a strong foundation of positive sending metrics.
Segmenting your "unknown" contacts is a sensible strategy. Keep them separate and only consider integrating them into your sending stream after your primary warming efforts are well-established and your domain's reputation is robust. This minimizes potential risks when your domain is most vulnerable.
If you do decide to send to a small batch of "unknown" addresses later in your warming process, monitor them intensely. Look beyond just bounces and spam complaints. Pay attention to engagement rates (opens, clicks), which are vital for building a positive sender reputation. For transactional emails, this means ensuring successful delivery of critical information.
Sending to 'unknowns' during warming
The risk is that these addresses might be problematic (e.g., inactive, deferred indefinitely, or even leading to timeouts) and negatively impact your new domain's reputation.
Reputation risk: Sending to unverified contacts during domain warm-up can cause performance issues.
Data quality: The "unknown" status itself suggests an issue with data quality from your validation service or the recipient server's responsiveness.
Segmenting 'unknowns' for later
Delaying sending to these addresses allows you to focus on building a solid reputation with your known, valid contacts first. Once your domain is warmed up and established, you can revisit these "unknown" contacts with less risk.
Controlled risk: Addresses with clearer statuses provide more predictable feedback, which is ideal during a new domain warming.
Monitoring key metrics and adapting strategy
Beyond the initial bounce rate, closely monitor your deliverability metrics, especially for transactional emails. If your ESP provides it, look for data on deferrals, open rates, and click-through rates. These indicators give a more complete picture of how mailbox providers are receiving your mail.
If you observe a consistent pattern of high deferrals or low engagement from a segment of unknown addresses, it may be prudent to remove them from your active sending list, at least for the duration of the warming process. It's better to send to a smaller, highly engaged, and validated list than to risk your reputation on uncertain contacts. Remember that a domain warming period can be a very long term thing.
Metric
Significance during warming
Action for 'unknown' status
Bounce rate
Direct indicator of invalid addresses. Keep this low.
If it remains low, good. Monitor for spikes if you send to unknowns.
Complaint rate
Crucial for sender reputation. Must stay near zero, especially for transactional mail.
Any increase when sending to unknowns means immediate stop.
Delivery rate (ESP reported)
Reflects acceptance by recipient servers. Can be lowered by deferrals.
If consistently low for unknowns, segment them out.
Open/click rates (if applicable)
Indicates recipient engagement. High engagement builds trust.
Low engagement from unknowns suggests they aren't worth the risk.
A cautious approach for long-term health
When facing "unknown" status emails after validation during transactional domain warming, it's prudent to exercise caution. While a low bounce rate and no spam complaints are positive, the lack of definitive delivery confirmation still poses a risk to your nascent sender reputation. The primary goal of warming is to establish trust with mailbox providers, and sending to uncertain contacts can undermine this effort.
I recommend segmenting these unknown contacts and focusing your warming efforts on the clearly validated, engaged users first. This ensures you're building a strong, positive sending history without unnecessary risks. Once your domain's reputation is well-established, you can carefully re-evaluate whether to incorporate a small, monitored portion of these unknown users.
Views from the trenches
Best practices
Always prioritize sending to email addresses with a confirmed 'valid' status during the initial warming phases.
Segment 'unknown' contacts and postpone sending to them until your core domain reputation is firmly established.
If you do send to 'unknown' addresses, monitor deliverability, deferrals, and engagement metrics meticulously.
Common pitfalls
Misinterpreting 'unknown' status as 'safe to send' and risking your new domain's reputation.
Not segmenting 'unknown' contacts, leading to slower warming or potential blocklist issues.
Failing to monitor engagement metrics beyond simple bounces and spam complaints.
Expert tips
"Unknown" status often means the validation service lacks sufficient data or faces server deferrals.
Some major mailbox providers, like Yahoo, intentionally defer responses to prevent quick validation.
Even if not an immediate bounce, high deferrals can still negatively impact sender reputation over time.
Expert view
Expert from Email Geeks says that 'unknown' status from hygiene services often indicates a lack of data or inability to verify, suggesting the service's limitations.
2024-01-28 - Email Geeks
Expert view
Expert from Email Geeks recommends continuing to mail these addresses, focusing on handling any bounces that occur.
Some mailbox providers, like Yahoo (and former Verizon/AOL properties), are known for their more aggressive deferral strategies, often accepting recipients during the initial SMTP handshake and only rejecting after the data is transmitted. This makes it challenging for email validation services to get an immediate, conclusive status.
