When can I include event attendees in an email warming list?
Michael Ko
Co-founder & CEO, Suped
Published 7 Jun 2025
Updated 16 Aug 2025
8 min read
Email warming is a critical process for establishing and maintaining a positive sender reputation. It involves gradually increasing your email sending volume to a new or cold IP address or domain, allowing mailbox providers to recognize your sending patterns as legitimate and trustworthy. The goal is to avoid being flagged as a spammer, which can lead to your emails landing in the spam folder or being rejected entirely.
A common question I encounter is whether to include event attendees, particularly those from webinars, virtual events, or physical tradeshows, in an email warming list. While these contacts have shown some level of engagement, their explicit consent to receive ongoing marketing emails needs careful consideration. Improperly adding them can quickly derail your warming efforts and damage your sender reputation.
The fundamentals of email warming
Email warming (or ramp-up) is the systematic process of sending emails in increasing volumes to build a positive sending history with Internet service providers. When you begin sending from a new IP address or domain, mailbox providers like Gmail and Outlook are cautious. They observe how your recipients interact with your emails, looking for positive signals like opens, clicks, and replies, and negative ones such as spam complaints or bounces. A proper warming strategy establishes trust, ensuring your messages reach the inbox.
Starting a warming list requires highly engaged, explicitly opted-in contacts. These are individuals who have clearly given you permission to email them, typically by signing up for your newsletter or service. Their consistent positive engagement signals to mailbox providers that your emails are desired and valuable, which is crucial for building a strong sender reputation.
The danger of adding less engaged or ambiguously consented contacts too early is that they are more likely to mark your emails as spam, leading to higher complaint rates. This can trigger spam filters, land your domain on a blocklist (or blacklist), and effectively undo all your warming efforts. For insights into how to build this engagement, consider looking at strategies to warm up a new email sender address.
Understanding event attendees and consent
Event attendees are a unique segment. They have interacted with your brand, but their consent for ongoing email marketing varies greatly depending on how their information was collected. Attending an event, whether virtual or physical, does not automatically grant you permission to add them to your general marketing lists. Their intent was to attend that specific event, not necessarily to receive future promotional emails.
Explicit consent means they actively checked a box or filled out a form specifically stating they wish to receive marketing communications. Implied consent, which often comes from event registration, means they provided their email for the event, but not necessarily for general marketing. Under regulations like GDPR and CCPA, relying solely on implied consent for broad marketing can lead to compliance issues and deliverability problems.
This distinction is critical. If attendees did not explicitly opt-in for marketing emails, including them in a warming list can quickly lead to high spam complaint rates. Mailbox providers interpret these complaints as a signal that your sending practices are unwanted, which can damage your sender reputation. For more on ensuring compliance, explore email marketing compliance after events.
Explicit consent
The gold standard for email marketing. Achieved when a user takes a specific, affirmative action to opt-in, such as checking an I agree to receive marketing emails checkbox during registration or signing up for a newsletter via a dedicated form. This indicates clear intent and a lower likelihood of complaints.
Implied consent
Often arises from business relationships or event attendance. While you may have their email for a specific purpose (like event access), they haven't explicitly agreed to ongoing marketing. Using these contacts for warming is risky due to a higher potential for spam complaints.
Varies by jurisdiction and initial agreement. Often implied for transactional, explicit for marketing.
Moderate to high. These contacts might be old, unengaged, or never explicitly opted into marketing communications.
Assessing the risk of event attendee data
Including event attendees who haven't explicitly opted into your marketing list poses a significant risk to your sender reputation. Regardless of whether it was a webinar or a physical event, without clear consent for ongoing marketing communications, these individuals are more prone to reporting your emails as spam. This can lead to your domain or IP being put on an email blocklist (also called a blacklist), severely impacting your deliverability. You can learn more about what happens when your email is blacklisted.
High complaint rates during warming can lead to irreversible damage to your sender reputation. Mailbox providers track these signals closely. If they see a sudden surge in complaints from a list that wasn't expecting your emails, they might label you as a sender of unwanted mail, making it difficult to reach the inbox even for your highly engaged contacts. It's often better to err on the side of caution and focus on truly opted-in users.
The goal of warming is to demonstrate consistent positive engagement. Introducing contacts who might not remember opting in, or who only intended to receive event-specific communications, directly contradicts this goal. Their lack of engagement or negative actions like spam reports can create a feedback loop that harms your deliverability for all future sends.
Virtual event/webinar attendees
These individuals registered online, often with a clear expectation of receiving event access details and perhaps a follow-up. Their digital interaction makes it easier to track their initial opt-in method. However, if they didn't explicitly check a marketing opt-in box, their expectation for ongoing emails is low, leading to higher complaint rates.
Primary concern: Lack of explicit consent for general marketing communications.
Risk: High spam complaints, potential for landing on a blocklist or blacklist, damaged sender reputation.
Physical event/tradeshow attendees
Contacts from physical events can be even riskier. While they visited your booth or attended a session, their email address might have been collected via a business card drop, a contest entry, or a sign-up sheet without clear marketing opt-in language. Memory of interaction might also be lower than for a digital event, increasing complaint risk.
Primary concern: Ambiguous consent and potential for outdated or poorly collected data.
Risk:High complaint rates, direct impact on sender reputation due to unexpected emails.
Strategic integration into your warming process
Given the risks, it's best to keep event attendees who haven't explicitly opted into marketing off your initial warming list. The warming process should be reserved for your most engaged and consented contacts to build a strong foundation of positive sender reputation. Once your domain and IP have a stable and high reputation, you can begin to consider a phased integration of these 'riskier' contacts.
When you do decide to integrate these lists, approach it with extreme caution and in very small increments. Segment these lists further based on recency of interaction and perceived engagement. For instance, an attendee from last week's webinar who downloaded a resource is less risky than someone who simply signed up for a tradeshow a year ago without further interaction. Even then, consider a re-engagement campaign with a clear opt-in prompt before adding them to regular sends. You can find out more about how to warm up an old email list if needed.
The key is to minimize negative signals. If you observe a spike in complaints, unsubscribes, or bounces after adding a segment of event attendees, immediately pause and re-evaluate your strategy for that segment. It's far easier to prevent damage to your sender reputation than to recover from it.
Complete initial warming: Ensure your IP and domain have a solid reputation with highly engaged, explicitly opted-in contacts.
Segment event attendees: Create separate segments based on how and when consent was obtained, and their level of engagement post-event.
Re-permission campaign: Send a specific, clear email asking them to opt into your marketing list. Include a compelling reason to subscribe and make it easy to say no. Only add those who explicitly opt-in.
Gradual integration: If you still choose to email those without explicit marketing opt-in, start with small batches and monitor metrics closely.
Maintaining a healthy sender reputation
Maintaining a healthy sender reputation requires ongoing vigilance. Even after successful warming, continuous monitoring of your email metrics is essential. Pay close attention to open rates, click-through rates, bounce rates, and especially spam complaint rates. A sudden increase in complaints can indicate issues with your list quality or content, regardless of how it was acquired. You can find a complete guide to email marketing for events for more comprehensive strategies.
Regular list hygiene is also crucial. Remove unengaged subscribers, hard bounces, and any addresses that generate complaints. This proactive approach ensures that you're consistently sending to an active and interested audience, which in turn reinforces a positive sender reputation and improves overall deliverability. Remember, quality over quantity is always the rule in email marketing.
Prioritizing permission and reputation
The decision to include event attendees in an email warming list should be approached with extreme caution. While these contacts have shown initial interest, the lack of explicit, clear consent for ongoing marketing communications makes them a high-risk group. Prioritizing explicit opt-in and maintaining a healthy sending reputation through careful list management and consistent monitoring is paramount. Sacrificing deliverability for list size is rarely a worthwhile trade-off.
Views from the trenches
Best practices
Always prioritize explicit opt-in for marketing emails, especially when building a warming list.
Segment event attendees based on how clear their marketing consent was, reserving the main warming list for clear opt-ins.
Consider a re-permission campaign for event attendees to explicitly confirm their interest in ongoing marketing.
Monitor email metrics closely, especially complaint rates, when introducing new segments to your sending.
Common pitfalls
Adding all event attendees directly to a warming list without verifying explicit marketing opt-in.
Ignoring the difference between implied consent for event communication and explicit consent for marketing.
Not segmenting event attendee lists, treating them all as equally engaged or consented.
Failing to monitor deliverability metrics like spam complaints after adding event attendee contacts.
Expert tips
Use clear language on event registration forms to obtain explicit consent for marketing communications.
If using a physical sign-up sheet, ensure it clearly states that signing up means agreeing to receive marketing emails.
For virtual events, embed a marketing opt-in checkbox that is unchecked by default.
Always prioritize list quality over quantity to maintain a strong sender reputation.
Marketer view
Marketer from Email Geeks says those populations are risky unless you are 200% sure they explicitly and voluntarily opted in to receive emails, because otherwise they may just complain, and this isn't good for warming up.
2022-02-03 - Email Geeks
Marketer view
Marketer from Email Geeks says that adding in risky contacts should only occur after the initial ramp-up, and then only in tiny increments to minimize any negative impacts.
Gmail and 
Outlook are cautious. They observe how your recipients interact with your emails, looking for positive signals like opens, clicks, and replies, and negative ones such as spam complaints or bounces. A proper warming strategy establishes trust, ensuring your messages reach the inbox.
