What is a volume spike and why is it problematic for deliverability?

A volume spike occurs when you send significantly more emails than your typical historical volume in a short period. ISPs perceive this as unusual behavior and may flag your emails as suspicious, potentially leading to higher bounce rates, spam folder placement, or even blocklisting. Maintaining a consistent or gradually increasing sending pattern is preferred.

Should I clean my list even if the maintenance email is critical for all users?

It's highly recommended to clean your list before sending a large maintenance email. Inactive or invalid addresses can lead to hard bounces and spam complaints, severely damaging your sender reputation. Even for critical updates, sending to a clean, engaged list first is a safer strategy, even if it means some very old contacts aren't reached immediately.

What are the risks of sending a large maintenance email without proper volume management?

For lists with low sending volume, a sudden large send can cause your domain or IP to be blocklisted by ISPs. This means your emails will be rejected or sent directly to spam. It can take considerable time and effort to recover your sender reputation and be removed from these blocklists, impacting all your future email campaigns.

Is it ever acceptable to send the entire large list at once for a maintenance email?

While immediate delivery of a large campaign might be desired, it's generally ill-advised for domains with low historical volume. The risk to your sender reputation and future deliverability is too high. A phased rollout, even if it takes a few days, is the best compromise to ensure the message eventually reaches its recipients without severe negative consequences.

What are the best practices for sending a large maintenance email campaign to a list with low sending volume? - Sender reputation - Email deliverability - Knowledge base

Sending a large maintenance email campaign to a list with low historical sending volume can be a significant challenge for email deliverability. Internet Service Providers (ISPs) like Google and Microsoft closely monitor sending behavior, and sudden, large spikes in volume can trigger spam filters, leading to poor inbox placement or even blocklisting.

The core issue lies in maintaining a consistent sender reputation. When your typical sending volume is low (e.g., around 1% of your total list), a sudden jump to emailing a million recipients signals an anomaly. This can be interpreted by ISPs as suspicious activity, potentially indicative of a compromised account or spamming behavior, even if the content is legitimate and necessary.

Successfully navigating such a campaign requires careful planning, adherence to email best practices, and a strategic approach to volume management. The goal is to deliver essential information without negatively impacting your domain's long-term deliverability.

Understanding sender reputation and volume spikes

The sudden increase in email volume from a low-volume sender to a large, unengaged list immediately raises red flags for ISPs. Their algorithms are designed to detect unusual sending patterns that often correlate with spam. Your sender reputation, built over time through consistent, positive sending behavior, can be severely damaged by a single mismanaged large send.

If you are on a shared IP pool, the risk is even higher. A significant volume spike could not only hurt your domain's reputation but also impact other senders sharing the same IP address. This can lead to the entire shared IP being placed on a blocklist (or blacklist), affecting all users on that pool. Conversely, if you have a dedicated IP, a sudden spike could still lead to your IP being blacklisted, requiring a recovery process.

The key is to avoid spikey behavior. ISPs prefer a steady, gradual increase in sending volume, often referred to as IP warming. A maintenance email, by its nature, often needs to reach a broad audience quickly, which directly conflicts with this principle. Therefore, a careful strategy is essential to balance the urgency of the message with the need to protect your sender reputation.

Strategic list management and segmentation

Before even considering sending, it is crucial to prepare your list. A large list with low sending volume likely contains many inactive or unengaged contacts. Sending to these addresses can result in high bounce rates, spam complaints, and even hit spam traps, all of which severely damage your sender reputation.

List cleaning: Thoroughly clean your list to remove invalid, inactive, or unengaged email addresses. If someone hasn't been successfully delivered to in the past two years, consider unsubscribing them or moving them to a highly re-engagement-focused segment. Even if they are paying customers, risking your deliverability for an unengaged contact can be detrimental.
Segmentation: Segment your list into smaller, more manageable chunks. Prioritize sending to your most engaged users first, as their positive interactions can help build trust with ISPs. Then, gradually expand to less engaged segments. This allows you to monitor deliverability metrics (opens, clicks, complaints, bounces) in real-time and adjust your strategy if issues arise.
Content relevance: Ensure the maintenance email content is highly relevant and clearly communicates its purpose. Avoid anything that could be perceived as spammy. Concise, clear messaging increases the likelihood of engagement, which is vital for deliverability.

While the urgency of maintenance communications is understood, a phased rollout is almost always necessary to protect your sender reputation when dealing with low prior volume and large lists.

Implementing a phased rollout

The primary challenge is distributing a large volume of emails without creating a volume spike. Here's how to approach it:

The challenge of immediate delivery

Sending a million emails at once, especially on a shared IP with low prior volume, will likely trigger spam filters. This can lead to a significant portion of your emails being rejected, sent to spam, or causing your sending IP to be blocklisted (or blacklisted).

Your sender reputation will suffer immediately, impacting future email campaigns far beyond this single maintenance send. This can be difficult to recover from, requiring extensive reputation building efforts.

Recommended phased rollout

Even for urgent maintenance, divide the list and send over several days, or even a week, depending on the list size and your normal volume. Gradually increasing your daily volume helps maintain your sender reputation and allows ISPs to adapt to your new sending patterns. Start by sending only 10-20% above your normal daily volume.

For very large lists (over a million), extending the send over 30 to 60 days, sending a segment each day, is a safer approach. This is similar to how initial email sending volumes are managed for new domains.

Consider setting up a recurring message, where each day, a segment of the list receives the maintenance email. This distributed approach, though slower, is far less risky for your long-term deliverability. You can also prioritize segments, sending to the most critical users first within the daily volume limits.

If immediate delivery to the entire list is absolutely non-negotiable due to legal or operational requirements, you might explore options like securing a pre-warmed dedicated IP or working very closely with your Email Service Provider (ESP) to discuss specific rate limits and a supervised burst. However, even with these measures, expect a potential hit to your reputation and be prepared for higher bounce and complaint rates.

Content, authentication, and monitoring

Beyond volume and list management, the content of your email and proper authentication are critical for deliverability, especially when sending to a dormant or low-engagement list.

Clear and concise subject lines: Clearly indicate that it's a maintenance notification. Avoid sensational or misleading language that could trigger spam filters or complaints.
Plain text version: Always include a plain text version of your email. Some email clients (and spam filters) prefer this, and it ensures readability even if the HTML doesn't render.
Unsubscribe option: Even for maintenance emails, include a clear and easy-to-find unsubscribe link. This is legally required in many regions and reduces the likelihood of spam complaints from recipients who no longer wish to receive communications.

Proper email authentication is non-negotiable. Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. ISPs rely heavily on these to verify the legitimacy of your emails. A misconfigured record can cause even legitimate emails to fail authentication and land in spam folders.

Finally, monitor your deliverability closely. Pay attention to bounce rates, complaint rates, and whether your IPs or domain appear on any blocklists or blacklists. Early detection of issues allows you to adjust your sending strategy and mitigate damage quickly. Regularly checking Postmaster Tools (for Gmail and Yahoo) is a good practice for domain reputation.

Views from the trenches

Best practices

Always segment your list, prioritizing engaged users for initial sends.

Gradually increase your sending volume to avoid sudden spikes.

Regularly clean your email lists to remove inactive or invalid addresses.

Ensure all email authentication records (SPF, DKIM, DMARC) are correctly set up.

Monitor deliverability metrics and sender reputation tools constantly.

Common pitfalls

Sending a large maintenance campaign to an entire dormant list at once.

Ignoring high bounce rates or spam complaints during mass sends.

Not having proper authentication (SPF, DKIM, DMARC) in place.

Failing to clean your list, leading to hitting spam traps.

Using misleading subject lines or content that triggers spam filters.

Expert tips

Communicate the urgency and importance of maintenance emails clearly.

Consider using a dedicated IP if large, infrequent sends are common.

Test email content and rendering across various email clients before sending.

Segment recipients by domain to avoid overwhelming specific ISPs.

Have a re-engagement strategy for critical but inactive contacts.

Expert view

Expert from Email Geeks says to first scrub the list thoroughly. If a recipient hasn't received emails successfully in two years, they should be unsubscribed or removed to maintain list hygiene.

2019-07-09 - Email Geeks

Expert view

Expert from Email Geeks says that after cleaning, the maintenance email should be randomly batched out in chunks every few hours to prevent sudden volume spikes.

2019-07-09 - Email Geeks

Navigating large maintenance campaigns effectively

Sending a large maintenance email campaign to a list with low sending volume is a delicate balancing act between urgency and deliverability. The most effective approach involves strategic list cleaning, careful segmentation, and a phased rollout to avoid shocking ISPs with sudden volume spikes. Prioritizing deliverability through these best practices ensures your critical communications reach the inbox without jeopardizing your sender reputation.

By understanding ISP behavior and proactively managing your sending patterns, even large-scale, urgent campaigns can be executed successfully. This prevents unnecessary blocklistings (or blacklistings) and maintains a healthy sending domain for all future communications.