Why would my emails appear to send from shared IPs when I have dedicated IPs?

This can happen due to various factors, including internal routing within the ESP's network, temporary use of shared pools during IP warm-up, or, as in the Sailthru/SparkPost case, reporting anomalies where internal filtering IPs are logged instead of the true external sending IPs. It's crucial to verify the actual sending IP via email headers.

What steps should I take if I see unknown IPs in my email logs or reports?

If you observe unexpected IP addresses, first check the full email headers of a test email sent from your platform. The Received header (specifically the bottom-most one) should reveal the true sending IP. Then, contact your ESP with your findings to clarify their routing and reporting mechanisms.

Does it matter if the volume on these shared IPs is considerably lower than my dedicated ones?

Even if the volume is low, any association with a blocklisted IP can potentially impact your sender reputation and deliverability. It's important to understand why these IPs are appearing and to monitor their status. If they're indeed used for your sending, even in a minor capacity, their reputation can affect yours.

When should I use a dedicated IP versus a shared IP for email sending?

Dedicated IPs offer greater control over your sending reputation and are generally recommended for consistent, high-volume senders. Shared IPs can be suitable for lower or inconsistent volumes, as the collective reputation of the pool provides some buffer. The choice depends on your sending volume, consistency, and ability to manage list hygiene.

How important is blocklist monitoring, especially when using dedicated IPs?

Regular blocklist monitoring is crucial to ensure your sending IPs and domains are not listed. A listing can cause your emails to be filtered to spam or rejected. Proactive monitoring allows for quick identification and remediation of any issues, protecting your overall email deliverability. Consider using a blocklist checking tool .

Why are Sailthru emails sending through shared IPs despite having dedicated IPs, according to SparkPost? - Troubleshooting - Email deliverability - Knowledge base

Recently, my team encountered a puzzling situation. We've invested in dedicated IPs for our email sending through Sailthru, expecting our messages to originate solely from those IPs. However, when checking our deliverability reports via SparkPost's monitoring tools, we noticed something unexpected: a number of our emails appeared to be sending through shared IP addresses. This immediately raised red flags, especially since these shared IPs were occasionally landing on email blocklists (or blacklists), like SpamCop.

It's a common concern for email marketers and deliverability professionals. When you pay for dedicated IPs, the expectation is clear: your email reputation is entirely yours to build and maintain, uninfluenced by other senders. Discovering that your messages are apparently routing through shared infrastructure can lead to worries about unwanted reputation impact and potential inbox placement issues. The volume on these shared IPs was considerably lower than our dedicated ones, but the fact that they were appearing at all and were tied to campaigns we didn't recognize was troubling.

This experience highlights a critical aspect of email deliverability: understanding how your Email Service Provider (ESP) manages your sending infrastructure and how their reporting reflects that. It's not always straightforward, and what you see in a report might not always tell the full story of where your emails are truly originating from.

Understanding dedicated versus shared IPs

The choice between dedicated and shared IPs is fundamental to an email sending strategy. Dedicated IPs provide complete control over your sending reputation, making them ideal for high-volume senders with consistent email practices. If your domain builds a strong reputation on a dedicated IP, it directly benefits your inbox placement. However, this also means any missteps, like sending to unengaged users or hitting spam traps, directly impact your sole IP.

Conversely, shared IPs pool the sending reputation across many users. This can be beneficial for lower-volume senders, as the collective good behavior of others in the pool can buffer against individual poor sending practices. However, the downside is that a few bad actors (senders of spam) in the shared pool can negatively affect everyone's deliverability, even if your own sending habits are impeccable. It's a trade-off between control and collective risk.

Understanding when to choose one over the other is crucial for your email program's success. Factors like sending volume, consistency, and your ability to manage recipient engagement play significant roles in this decision. For more on this, you can explore the differences between shared and dedicated IPs.

Dedicated IP characteristics

Reputation control: Your sending practices solely dictate your IP reputation. A bad sender can ruin their own IP, but no one else's.
Volume requirement: Best for high-volume senders with consistent email streams to maintain a good sending history.
Warm-up period: Requires a careful IP warm-up process to establish trust with mailbox providers.

Shared IP characteristics

Reputation impact: Reputation is influenced by all senders on the IP. Good senders help bad ones, and vice versa.
Volume flexibility: Suitable for low to moderate volume or inconsistent sending, as the pool's overall activity maintains reputation.
No warm-up: No need for an IP warm-up period, as the IP is already established by the collective sending.

How ESPs manage dedicated and shared IP pools

Email Service Providers (ESPs) like Sailthru, which partners with

SparkPost for delivery, employ complex systems to route and send emails. Even when you have dedicated IPs, there are various scenarios where shared IPs might appear in your logs or monitoring reports. This can be due to internal routing, fallback mechanisms, or even how the ESP's monitoring platform interprets the data.

For instance, during an IP warm-up phase, some ESPs might temporarily route overflow volume through a shared IP pool to prevent overburdening the new dedicated IP. This helps to gradually build the dedicated IP's reputation. Another possibility is that certain types of transactional emails or internal notifications might default to shared infrastructure, even if your marketing emails use dedicated IPs.

SparkPost, like other major ESPs, has detailed documentation on how they manage dedicated IP pools. It's worth reviewing this to understand their standard practices. Sometimes, the issue isn't with the sending itself, but with how monitoring tools capture and attribute IPs.

Diagnosing unexpected IP appearances

In our specific case with Sailthru and SparkPost, the situation turned out to be a reporting anomaly. The SparkPost product team confirmed that their platform was picking up internal filtering IPs rather than the actual external IPs the mail was being sent on. This meant our emails were indeed going out on our dedicated IPs, despite what the initial reports suggested. It was a relief to get this clarity, as it meant our dedicated IP strategy was sound.

This highlights a crucial point: unexpected data in your deliverability reports doesn't always indicate a problem with your sending setup. Sometimes, it's a quirk in the reporting system itself. This is especially true with the complex infrastructure used by large ESPs. If you see discrepancies, the first step should always be to reach out to your ESP's support or technical team for clarification.

To verify the actual sending IP in such situations, you can often check the email headers of a received message. The Received headers will show the path the email took and the originating IP address. This provides a direct confirmation of which IP was used for delivery. You can also explore methods to check your sending IP without ESP contact.

Checking email headers for sending IP

When troubleshooting unexpected IP addresses in your reports, inspecting the full email headers is key. Look for the Received header lines. The bottom-most Received header typically indicates the IP address from which the email originated. This is often the IP that matters most for deliverability and reputation.

Example email header snippet

Received: from [actual.sending.ip] (HELO mail.example.com) by [receiving.mail.server.ip] with ESMTP id ABCDE for <recipient@example.com>; Mon, 1 Jan 2023 12:00:00 -0000

Impact of blocklists and monitoring

One of the primary concerns when seeing unexpected shared IPs is the risk of being listed on a blocklist (or blacklist). These lists are compiled by various organizations to identify and track IP addresses with a history of sending spam or unsolicited mail. If an IP address, whether shared or dedicated, lands on a major blocklist, it can severely impact email deliverability, leading to messages being rejected or sent directly to the spam folder by mailbox providers.

Even if the shared IPs observed in reporting are internal or for low-volume background tasks, the concern about a blocklist listing is valid. Monitoring these lists is a proactive measure to safeguard your sending reputation. If your IP, or any IP associated with your sending, gets listed, it requires immediate action to understand the cause and request delisting.

Regularly checking common blacklists and blocklists, such as SpamCop (which was mentioned in my experience) and those used by major mailbox providers, is a critical part of a robust deliverability strategy. Automated blocklist monitoring services can alert you instantly if your IPs or domains are listed, allowing for swift intervention and mitigation of potential damage. You can also learn more about how email blacklists actually work.

Blocklist Type	Impact	Example
Public Blacklists	Widely used by mail servers to filter out spam. Listing can lead to significant delivery issues.	SpamCop, Spamhaus
Internal/Private Blocklists	Managed by individual mailbox providers (e.g., Gmail, Outlook). Can lead to messages going to spam folders or being rejected for their users.	Gmail's internal filter, Outlook's SmartScreen

Views from the trenches

Best practices

Ensure clear communication with your ESP about their IP management and reporting practices.

Regularly monitor your email logs and deliverability reports for any unexpected anomalies.

Verify sending IPs via email headers for crucial campaigns, especially after configuration changes.

Implement a robust blocklist monitoring system to quickly detect and address any listings affecting your IPs or domains.

Common pitfalls

Assuming unexpected IPs in reports indicate a sending issue without further investigation.

Neglecting to ask your ESP for clarification on IP routing and reporting specifics.

Failing to review email headers to confirm the actual sending IP for delivered messages.

Ignoring shared IP blocklist warnings, even if volume is low, as they can still impact overall reputation.

Expert tips

For complex ESP setups, request detailed diagrams of their mail flow for your account.

If using multiple sending platforms, consolidate monitoring to a single dashboard for a holistic view.

Consider segmenting email types (e.g., transactional, marketing) to different dedicated IP pools for better control.

Engage with your ESP's deliverability team for regular reputation reviews and proactive advice.

Marketer view

A marketer from Email Geeks says they can see the volume on the seemingly random IPs. It's considerably lower traffic compared to the dedicated IPs.

2020-09-30 - Email Geeks

Marketer view

A marketer from Email Geeks says SparkPost provides visibility into the subject lines of various email campaigns from these shared IPs, and they are definitely not campaigns sent by their company.

2020-09-30 - Email Geeks

Key takeaways for email deliverability

The experience of seeing shared IPs in reports despite having dedicated ones highlights the complexity of modern email sending infrastructure. While it can be alarming, it's often a case of reporting discrepancies rather than a fundamental flaw in your dedicated IP setup. It underscores the importance of not just monitoring, but also deeply understanding your ESP's systems and having open lines of communication with their support teams.

Maintaining a strong email deliverability posture requires constant vigilance, from understanding IP behavior and managing sender reputation to proactively monitoring for blocklist listings. By asking the right questions, verifying information, and leveraging the tools at your disposal, you can ensure your email program continues to reach the inbox effectively.