How to check the opening of a letter sent via mail()?

Question

PHP

Friends, for the purpose of a flexible marketing policy, we are writing our own class of e-mail newsletters.
Tracking the transition from the letter is not difficult, UTM tags help us out here.
And how to track that the letter was opened? How does MailChimp and others like it do it? We need to collect statistics for analysis.

Answer 1

There is no 100% reliable way. The one that Vladimir described covers ~ 80% of the possibilities. Except when:
1. Mailers cache access to pictures. Simply put, even before the user receives the letter, the mail bot will first go there to save the picture to itself, and already show it from itself. As a result, the letter is marked on your side as read, but in fact it is not known whether it got into the inbox / spam at all, or was filtered out by system / user filters.
2. Mailers now also fire and cut out tracking links, maybe they need to be somehow better masked, but they often cut me out simply, because of which the statistics were not accurate.
3. Many features depend on the configuration (dkim, spf, dmarc, etc) and the reputation of the sending domain. If the reputation is good for a long time, it is even possible to use micro-markup in emails, like this:
In general, there are a lot of factors.

Answer 2

Insert a link to an image, even if it's a pixel with a unique token corresponding to the given email, and track when it's accessed.

Answer 3

I wrote an article: https://habrahabr.ru/post/230063/. Now I use Sidekick - it doesn't always work. But only DRM can really give guarantees, but this technology does not make sense to use for advertising. Look at how it works in ToutApp - according to the results of the mailing list, this service gave clear statistics.