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Telegram, WhatsApp, SimpleX, WeChat and similar
All the ones you mentioned have the option to mark as unread.
I was not aware of it, I will look for more information on this news.
by default I disable the read receipt on Thunderbird
In the second article, a method for identifying email reads through companies is mentioned. Take a look.
First of all, I would like to point out that the article only talks about emails read from the smartphone, then I suggest you give a read to this nwes.
I was already using this system twenty years ago. An image hosted on my site that stored all the IPs that displayed it.
For this reason, both the Thunderbird that I use on my computer and the email client that I use on my android phone, do not load images by default, but only if I authorize them.
At least, this is the information in my possession, in case it is wrong, I will try to inquire about it.