Do ISPs frequently re-fetch email tracking images?

Generally, ISPs do not continuously re-fetch email tracking images. They usually fetch an image once and then cache it. Re-fetches typically occur if the email is opened on a different device or email client, or if the cached image expires, prompting a new request to the server.

What are the primary causes of delayed email opens?

Several factors cause delayed email opens. These include technical issues like greylisting or ISP throttling, network congestion, large email sizes, and recipient behavior (e.g., opening an email much later). Sometimes, security scanners or proxy servers can also cause initial fetches that appear as 'opens' before the recipient actually sees the email.

How do ISP image proxies and caching affect email tracking pixel accuracy?

Email tracking pixel accuracy is often affected by ISP image proxies and caching. These systems pre-fetch images, including tracking pixels, which can register an 'open' before a human recipient views the email. Additionally, recipients blocking images by default can lead to underreported open rates. This means reported open rates may not always perfectly reflect true human engagement.

Does a high number of delayed opens indicate a deliverability problem?

Delayed opens, particularly if consistent and widespread, can indicate underlying deliverability issues like being caught in spam filters (or on a blocklist/blacklist) or heavy throttling. However, individual delayed opens often reflect genuine recipient behavior, such as opening an email on a different device or revisiting it later. It's crucial to analyze patterns rather than single occurrences.

Do ISPs re-fetch email tracking images, and what causes delayed email opens? - Troubleshooting - Email deliverability - Knowledge base

Email open tracking is a common practice for marketers and businesses to gauge recipient engagement. It typically relies on a tiny, invisible 1x1 pixel image embedded in the email. When the email is opened, this pixel is loaded from a server, registering an "open" event. However, this seemingly straightforward process can lead to confusion, especially when you observe unexpected open data, such as opens registered days or even weeks after an email was sent.

The core questions often revolve around whether Internet Service Providers (ISPs) re-fetch these tracking images and what underlying factors might contribute to a delayed email open. These are critical aspects to understand for accurate email analytics and effective deliverability strategies.

Understanding how ISPs handle tracking images and the common causes of delayed email opens can shed light on discrepancies in your metrics and help you refine your sending practices. It's a nuanced area where technical behaviors intersect with user habits.

How email open tracking works

Email open tracking functions by embedding a small, transparent image, often called a tracking pixel, into the HTML of an email. Each pixel has a unique URL. When a recipient opens an email and their email client downloads images, a request is sent to the server hosting that pixel. This request is then logged as an open.

Many modern email clients, such as

Gmail and

Outlook, employ image proxy servers. This means that when an email is received, the ISP's servers may download and cache all images, including tracking pixels, before delivering the email to the recipient's inbox. This pre-fetching can sometimes lead to opens being recorded even if the recipient hasn't actually viewed the email yet. For more details on this, you can learn how email open rates are affected by caching.

This pre-fetching behavior, while designed to improve recipient privacy and security by stripping potentially malicious code and obscuring IP addresses, also means that the initial pixel load may not correspond to an actual human open. Therefore, while open tracking provides valuable insights into engagement, its accuracy can be imperfect due to these technical intermediaries. To gain a deeper understanding of the mechanism, consider this resource on how email open tracking works.

Do ISPs re-fetch tracking images?

In most cases, ISPs (or rather, their proxy servers) will only fetch an email tracking image once for caching purposes. Once the image is cached, subsequent views of the same email on the same client will likely load the image from the cache rather than re-fetching it from your server. This is efficient for the ISP and generally good for user experience.

However, there are specific scenarios where an image might be re-fetched. If a recipient opens the same email on a different device or email client, that new client will typically fetch the image independently. For example, opening an email on a desktop client like

Apple Mail and then later on a mobile device would result in two distinct image fetches, provided the mobile client doesn't use the same proxy or cache. Additionally, cached images have an expiration, and once that time is up, the image might be re-fetched if the email is opened again.

For instances where you see opens from emails sent long ago, it's highly probable that the recipient is genuinely re-opening or viewing the email, possibly on a different device, or the cached image has expired. Machine-generated image loads (from bots or security scanners) usually occur very close to the time of receipt, not weeks later. Some ISPs, like Yahoo, also employ their own caching mechanisms, further complicating precise open tracking.

Unique tracking pixels

Ensure each email sent has a unique tracking pixel URL for accurate individual open tracking. This helps differentiate new opens from cached loads and avoids overcounting. If the pixel URL remains the same for every send, it becomes impossible to distinguish unique opens from re-fetches or cache hits.

Direct image loading

Process: Email client fetches images directly from the sender's server.
Open accuracy: More accurate reflection of a human open, as pixel loads immediately signal viewing.
Privacy: Recipient's IP address might be exposed to the sender.

Image proxy loading

Process: ISP's proxy server fetches and caches images, then serves them to the client.
Open accuracy: Can result in false opens due to pre-fetching; less precise.
Privacy: Recipient's IP address is masked by the proxy, enhancing privacy.

What causes delayed email opens?

Delayed email opens can be attributed to several factors, ranging from technical issues on the sending or receiving end to recipient behavior. One common technical cause is greylisting, where a receiving mail server temporarily rejects an email from an unknown sender. The sending server must then retry, causing a delay.

Another factor is ISP throttling, where email providers limit the number of emails a sender can send within a specific timeframe, especially for new or suspicious senders. This is a common practice to combat spam and protect users. For more information, explore what email throttling is. Recipient-side issues, such as slow internet connections, antivirus scans delaying mail delivery, or simply a user not checking their email frequently, can also lead to delayed opens. For instance, Yahoo inbound emails have been known to experience delays.

Additionally, large email sizes, especially those with numerous high-resolution images or attachments, can take longer to download, contributing to delays. Server-side problems, like high server load or misconfigurations at the ISP's end, can also queue messages, causing them to be processed and delivered slowly.

Cause of delay	Description	Impact on opens
Greylisting	Temporary rejection of emails from unknown senders, requiring retry.	Initial delivery delay, leading to a later recorded open.
ISP throttling	ISPs limit emails from specific senders to prevent spam.	Emails are queued, causing significant delays in delivery and opening.
Recipient behavior	User opens email days or weeks after receipt.	Open logged much later than the send time, accurately reflecting user action.
Large email size	Emails with many images or large attachments take longer to download.	Open recorded once the content, including tracking pixel, fully loads.

Example of an email header showing delays

Received: from [sender.domain] ([sender.ip]) by [recipient.mailserver] with ESMTPSA id [message.id] for <recipient@domain.com>;
	[Date and time of initial receipt]
Received-SPF: pass ([recipient.mailserver]: domain of sender.com designates sender.ip as permitted sender) client-ip=sender.ip;
X-Mailer: YourEmailService
Message-ID: <unique.message.id@sender.domain>
Date: [Original message send date]
Subject: Your important update
To: recipient@domain.com
From: "Sender Name" <sender@domain.com>

--[boundary_string]
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<html><body>
...<img src="https://tracking.domain/pixel?id=unique_id" width="1" height="1">...
</body></html>
--[boundary_string]--

Impact on deliverability and accuracy

The prevalence of image proxies and the nature of email caching mean that raw email open rates are not always a perfect indicator of true engagement. Pre-fetched opens or opens from cached images can inflate numbers, while image blocking by recipients can lead to undercounting. It's important to recognize that a registered open doesn't always equate to a human recipient viewing the email at that exact moment. Understanding the impact of tracking pixels on deliverability is key.

Delayed opens, while sometimes indicating technical hurdles like greylisting or throttling, often reflect genuine recipient behavior. A user might open an email on a desktop in the morning, then revisit it on their phone later, or discover an old email in their inbox and open it days or weeks after it was sent. While a singular delayed open isn't a red flag, consistently seeing significant delays could indicate broader deliverability challenges or a need to re-evaluate your audience's engagement patterns. Learn if delayed email opening impacts deliverability.

ISPs (Internet Service Providers) track various engagement signals beyond just opens, including clicks, replies, and even deletions without opening. Therefore, while open rates provide a baseline, a holistic view of engagement is crucial for maintaining a strong sender reputation and ensuring your emails consistently reach the inbox, rather than being caught by spam filters (or blocklists). This external article highlights how ISPs and spam filters work.

Views from the trenches

Best practices

Monitor open rate trends over time, not just individual opens, to identify patterns.

Segment your audience based on engagement levels to tailor content and sending frequency.

Implement DMARC authentication to improve trust and deliverability with ISPs.

Common pitfalls

Over-reliance on open rates as the sole measure of email campaign success.

Ignoring delayed opens, which can mask underlying deliverability issues or signal re-engagement.

Not understanding ISP caching and proxy behavior for image loading.

Expert tips

Analyze engagement beyond opens, including clicks, conversions, and replies.

Consider a multi-metric approach to evaluate campaign performance comprehensively.

A/B test different email formats to see how they impact open and engagement rates across various clients.

Expert view

Expert from Email Geeks says pre-fetched images will be cached and will expire from the cache, so they are not re-fetched constantly.

2024-02-15 - Email Geeks

Expert view

Expert from Email Geeks says if pre-fetching is done by the mail client, and the user opens the same email on a different client, the image will be re-fetched.

2024-02-15 - Email Geeks

Navigating email open complexities

The landscape of email deliverability is complex, and understanding how ISPs handle tracking images and what leads to delayed opens is crucial for accurate analytics. While ISPs typically cache tracking images and do not repeatedly re-fetch them, various factors can cause an image to be loaded again, or for an open to be genuinely delayed.

By focusing on a holistic view of engagement metrics and continuously optimizing your sending practices, you can ensure better inbox placement and more reliable data, even with the inherent complexities of modern email systems. Always monitor your deliverability and adjust your strategies based on observed patterns.