Do ISPs re-fetch email tracking images, and what causes delayed email opens?

Email marketer from SendGrid Blog shares that tracking pixels (tiny, transparent images) embedded in emails are used to detect when an email is opened. When the email is opened and images are displayed, the tracking pixel is loaded from a server, triggering an open notification. Delayed opens can occur when recipients open emails much later, or when images are not immediately displayed due to settings or connection issues.

Email marketer from Reddit user u/johndoe123 shared in a reddit thread that some people have the habit of opening emails from weeks or months ago. This accounts for some delayed opens. Others are because of mobile devices that download emails when in wifi zones but don't load images until later.

Email marketer from EmailOctopus Blog mentions that delayed opens can happen if a recipient opens the email offline, and the images are loaded (and the tracking pixel fired) when they reconnect to the internet. Also, some people simply open old emails much later than expected.

Marketer from Email Geeks shares the idea of plotting a histogram of opens from x days since the email was sent. There will generally be a sharp decline, then a long tail of opens even months later, depending on send frequency and email content.

Email marketer from ActiveCampaign Blog shares that email opens are measured by a small, invisible image (a tracking pixel) in the email. When the recipient opens the email and their email client loads images, it sends a request to the server hosting the image, which is recorded as an open. A delayed open could simply be that the user opened the email at a later time.

Email marketer from Campaign Monitor shares tips for improving email engagement, noting that some recipients open emails long after they were sent. This can be due to various reasons, such as delayed image loading or simply the recipient getting around to reading the email later. This delayed engagement will still trigger the tracking pixel.

Email marketer from HubSpot Blog identifies that email open rates can be impacted by various factors, including the recipient opening emails at a later date, or when the tracking pixel is blocked. Also, sometimes an email looks unread, but it was actually read.

Email marketer from StackOverflow user email_guru explains that the typical method for tracking email opens is to embed a 1x1 pixel transparent GIF image in the HTML email. When the email is opened, the email client requests this image from the server, and that request is logged as an open. Delayed opens are when this image is requested later.

Email marketer from Mailgun Blog explains that ISPs and email clients often cache images to reduce bandwidth consumption. This caching mechanism can sometimes lead to inaccurate open tracking if the same image is used across multiple emails and a recipient opens a previous email containing that image, it might register as a new open.

Email marketer from Email on Acid highlights that open rates aren't always a perfect metric due to limitations with tracking pixel technology. Opens can be influenced by recipients opening emails much later, or when images are not displayed by default. Many clients won't load images automatically and require the user to click accept, and some people will never do this.

Email marketer from Gmass notes that not all email opens are accurately tracked. Factors such as image blocking and privacy settings can prevent the tracking pixel from loading, resulting in an underestimation of open rates. However, if images are loaded later, delayed opens will be recorded.

Expert from Email Geeks explains that pre-fetched images are cached but will expire. If a user opens the same email on a different client, the image will be refetched. Image loads at times significantly after receipt are likely due to user action.

Expert from Word to the Wise explains that many mailbox providers now proxy images to protect the privacy of their users. This can result in all image fetches appearing to come from the proxy server's IP address. ISPs also cache these images and may refetch depending on their cache policies.

Expert from Spam Resource explains that open tracking with images is not reliable. Some people do not load images automatically so an open event is missed. Some clients may prefetch images, causing an open to be tracked when the email is not actually opened by the recipient. Open tracking is not reliable and can lead to false positives.

Documentation from Oracle Help Center details that the open rate metric reflects when an email's tracking pixel is downloaded. This download typically occurs when the recipient views the email and their email client downloads images. Delayed opens often reflect a user opening an email later. Caching will impact results but is generally short term.

Documentation from ietf.org explains that HTTP caches are typically used by mail user agents to minimize network traffic and improve the user experience. These caches will store images fetched by the MUA and reuse them when the same image is encountered again, either in the same message or in a different message. Caches are expected to be consistent with HTTP caching semantics.

Documentation from Litmus explains that email open tracking isn't always precise due to image blocking (images aren't displayed by default), text-only email clients (no image loading), and email client pre-fetching. Also, email clients might cache images aggressively, so tracking is complicated.

Documentation from Mozilla explains that browsers and other HTTP clients (like email clients) implement caching mechanisms. These caches store resources (like images) to reduce latency and network traffic. When a resource is requested, the cache is checked first, and if the resource is found, it's served from the cache. This can lead to a tracking pixel being served from a local cache and not recording a new open if not configured properly.

Documentation from SuperOffice explains that the technology behind email tracking involves embedding a tiny, invisible image within the email content. When a recipient opens the email and their email client loads the image, a request is sent to the server hosting the image. This request is then recorded as an email open. Delayed requests will still trigger an open, even if late.