How do you know if a letter has been opened?

Question

Email

I know that there are services for sending messages to subscribers, which show how many letters have been viewed.
First question: is it possible to send to one person and find out if he has viewed the letter?
The second question is: is it possible to implement this yourself?

Answer 1

1. You can. But no guarantee it will work.
2. You can. But no guarantee it will work.
Now for the implementation.
There are standard methods - check "Delivery notification", "Read notification". There are non-standard - for example, a picture in the signature, loaded from the site. It doesn't have to be a pixel - you can insert a completely legitimate signature with a logo.
But!
The reaction to alerts can be disabled - for example, TB allows you to do this, Outlook does not. And if the user is asked the question "send a read receipt" it is not at all a fact that he will agree to send it.
The display of pictures in the letter can be disabled - and in TB it is disabled by default. In addition, the letter can generally be converted to plain text.
As a result, the answer will look like this - it is possible, but there is some guarantee that it will work only when sent to servers like mail.ru, where the user has limited options for processing the letter. On corporate mail, they can do anything with it (mail), and there the guarantee tends to zero.

Answer 2

It was already:
How to check the opening of a letter sent via mail()?