How can I send 2 million transactional emails in a day without damaging sender reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 1 May 2025
Updated 18 Aug 2025
8 min read
Sending 2 million transactional emails in a single day is a significant undertaking that requires careful planning to protect your sender reputation. While transactional emails, by nature, often have higher engagement rates than marketing emails, such a large volume can still trigger red flags with internet service providers (ISPs) if not handled correctly. My primary goal in such a scenario is always to ensure those crucial messages reach the inbox without causing long-term damage to the sending domain or IP.
The key to success lies in understanding the nuances of transactional email, adhering to best practices, and leveraging robust infrastructure. It is not merely about having the technical capacity to send millions of emails, but about doing so in a way that signals legitimacy and value to mailbox providers.
This guide will walk you through the essential steps and considerations to achieve high-volume transactional email delivery without compromising your hard-earned sender reputation.
Differentiating transactional from bulk mail
First, let's clarify what truly constitutes a transactional email. Many senders mistakenly classify bulk notifications or updates as transactional, simply because they aren't direct marketing. However, mailbox providers have a much stricter definition. A true transactional email is typically a message triggered by a recipient's specific action, like a password reset, order confirmation, or shipping notification. These emails are expected and generally have high engagement.
If the 2 million emails you plan to send are, for example, annual privacy notices or general account updates that are not explicitly requested by the user, they might be perceived by ISPs more like bulk (or marketing) emails. This distinction is critical because bulk mail is held to different standards, particularly regarding unsubscribe options and complaint rates. Sending non-true transactional emails without an unsubscribe link can lead to significant deliverability issues and damage your sender reputation.
The true meaning of transactional
Be very clear on the nature of your emails. If the recipient did not explicitly trigger the message through a direct interaction, it is not a transactional email in the eyes of ISPs. Misclassifying emails can lead to higher spam complaint rates, lower engagement, and ultimately, damage to your sender reputation. For instance, sending unsolicited mail will likely land your message exclusively in the junk folder, severely lowering your sender reputation. Ensure compliance with regulations like the CAN-SPAM Act, which mandates unsubscribe mechanisms for commercial emails.
Even if your emails are genuinely transactional, a sudden, massive surge in volume can appear suspicious to ISPs. They look for consistent sending patterns. If your typical daily volume is, for example, 100,000 emails, jumping to 2 million in one day is a significant deviation that can trigger throttling or outright blocking, regardless of content.
Sender reputation and authentication
For high-volume sending, especially when dealing with millions of emails, robust email authentication is non-negotiable. This includes SPF, DKIM, and DMARC. Properly configured records for these protocols verify that your emails are legitimate and prevent spoofing. ISPs heavily rely on these to determine if an email is trustworthy. For example, Outlook and Gmail now explicitly require senders of bulk mail (5,000+ messages per day) to authenticate their emails with SPF, DKIM, and DMARC for proper deliverability. Without these, your 2 million emails are highly likely to be rejected or sent to spam.
Beyond authentication, your sender reputation is paramount. This is influenced by factors such as bounce rates, spam complaint rates, and engagement (opens and clicks). For transactional emails, you'd typically expect low bounce rates and spam complaints because the recipients anticipate these messages. However, even legitimate transactional emails can face issues if your lists are not clean or if there's a significant spike in sending volume from a cold IP.
If you are sending from a new domain or IP, or if this 2 million email send represents a massive increase from your usual volume, an IP warming (or domain warming) strategy is crucial. This involves gradually increasing your sending volume over time, allowing ISPs to build a positive reputation for your IP address and domain. Attempting to send 2 million emails on a cold IP will almost certainly result in high bounce rates and spam classifications, severely damaging your sender reputation. While there is no universal number, some recommend staggering email sends to improve sender reputation.
Good practices for reputation
Authentication: Implement SPF, DKIM, and DMARC correctly for all sending domains.
Low engagement: Sending unwanted or irrelevant emails, even if transactional.
Spam traps: Sending to purchased or old lists that contain spam traps.
Infrastructure and sending strategy
To handle 2 million emails in a day, you will need a robust email sending infrastructure. This typically means partnering with a reputable transactional email service provider (ESP) rather than attempting to send from your own servers without specialized expertise. These services are built to handle high volumes, manage IP reputation, and ensure high deliverability rates. They often offer dedicated IPs, which is crucial for such scale, as it gives you full control over your sending reputation.
When selecting an ESP, consider their ability to scale, their deliverability track record, and the level of control they provide over sending volumes and speeds. Some ESPs specialize in transactional email and have strong relationships with ISPs, which can be invaluable when sending at this scale. They can help you with managing large sending volumes effectively.
A crucial aspect is the domain used for sending. Always use a dedicated subdomain (e.g., mail.yourdomain.com) for transactional emails, separate from your main marketing or corporate domain. This isolates the reputation of your transactional sends. If, for any reason, your transactional emails encounter issues (e.g., higher-than-expected complaints), the reputation of your main brand domain remains unaffected. This isolation strategy is a fundamental best practice for high-volume senders.
Furthermore, ensure that your email content is clean and well-structured. Avoid broken links, unnecessary images, or anything that might trigger spam filters. Although transactional emails are less scrutinized for content than marketing emails, maintaining a high-quality format contributes to a positive sending profile. This also includes using a recognizable sender name and avoiding 'no-reply' addresses where possible, to encourage positive engagement.
Monitoring and recovery
Even with the best preparation, continuous monitoring of your email deliverability is essential, especially when sending at scale. You need to track key metrics like bounce rates, spam complaint rates, and open rates in real time. Most reputable ESPs provide dashboards for this. If you see a sudden increase in bounces or complaints, it indicates a potential issue that needs immediate attention. High bounce rates, particularly those exceeding 2%, are considered a red flag by ISPs.
Actively monitor for any instances of your IP or domain being placed on a blocklist (or blacklist). Blocklists are databases of IP addresses or domains with a poor sending reputation, and inclusion can severely hinder your email delivery. While transactional email services typically handle blocklist monitoring, it's prudent to conduct periodic blocklist checks yourself. If you do find your IP or domain on a blocklist, follow the necessary delisting procedures promptly. Remember that email blacklists work by providing data to ISPs who then make their own blocking decisions.
Beyond quantitative metrics, leverage feedback loops (FBLs) provided by major ISPs. FBLs notify you when a recipient marks one of your emails as spam. This allows you to quickly identify and remove problematic subscribers, preventing future complaints and protecting your reputation. Ignoring FBLs is a common reason for sustained deliverability issues. Additionally, regularly checking your Google Postmaster Tools data is crucial, as it provides insights into your reputation specifically with Google properties.
If, despite your best efforts, your sender reputation takes a hit, it is crucial to act swiftly. This might involve pausing sends, identifying the root cause (e.g., a sudden increase in spam complaints or bounces), cleaning your lists, and then slowly rebuilding your reputation with a gradual increase in sending volume. Maintaining a positive domain reputation is an ongoing process.
Continuous deliverability assessment
Metric
What it tells you
Target threshold
Bounce rate
Percentage of emails that couldn't be delivered.
< 0.5% (hard bounces) / < 2% (total bounces)
Spam complaint rate
Percentage of recipients marking your email as spam.
< 0.1%
Open rate
Percentage of recipients who open your email. High for transactional.
Varies, but high for transactional emails
Blocklist status
Whether your IP or domain is listed on a blocklist.
Zero listings
Maintaining excellent sender reputation when sending 2 million transactional emails requires a proactive and strategic approach. It's an ongoing process of adhering to best practices, utilizing the right infrastructure, and diligently monitoring your performance to ensure optimal deliverability.
By focusing on proper authentication, a smart sending strategy, and continuous monitoring, you can confidently deliver your high volume of transactional emails without risking your sender reputation or email deliverability.
Views from the trenches
Best practices
Always verify that emails are truly transactional, triggered by explicit user actions.
Utilize a dedicated subdomain for transactional sending to protect your primary domain's reputation.
Implement a gradual IP and domain warming process for any new sending infrastructure or significant volume increases.
Ensure all email authentication protocols like SPF, DKIM, and DMARC are correctly set up and aligned.
Common pitfalls
Misclassifying bulk notifications as transactional, leading to higher spam complaints.
Attempting to send a massive volume of emails from a cold IP without proper warming.
Neglecting email authentication, which results in emails being rejected or sent to spam folders.
Ignoring feedback loops and deliverability reports, allowing problems to escalate unnoticed.
Expert tips
Engage legal counsel to confirm email content compliance with privacy regulations before large sends.
Consider a high-volume transactional ESP that offers granular control over sending speeds and dedicated IPs.
Review sender reputation consistently using tools like Google Postmaster Tools.
Maintain pristine list hygiene by regularly removing inactive or invalid addresses.
Marketer view
Marketer from Email Geeks says understanding the client's usual daily sending volume is crucial before attempting a 2 million email send.
2019-12-04 - Email Geeks
Marketer view
Marketer from Email Geeks says if the email content is genuinely transactional and the volume isn't a sudden shock to the infrastructure, the send should be fine.
Gmail now explicitly require senders of bulk mail (5,000+ messages per day) to authenticate their emails with SPF, DKIM, and DMARC for proper deliverability. Without these, your 2 million emails are highly likely to be rejected or sent to spam.
Google properties.
