Short answer: No.
Longer answer:
The feature to load remote content only after the user allowed it, is mostly implemented to protect your privacy. Of course there are effects like less data usage and maybe some people do not want to have so many images in an e-mail, but the main reason for this feature is data protection.
When there would be a way around this (a feature for you as sender and person who wants to track the e-mail), this would be a bug in the protection feature and the mail program developer would hopefully fix it as soon as possible.
So what you're asking for is a hack or exploit, that allows you to circumvent security measures in the client software. There may be answers for specific softwares, but all of them would be security bugs in the software and may get closed sooner or later.
Another remark:
Is there a way to track emails undetectably, so no one can detect that emails are being tracked?
Don't you think, that this behaviour is kind of sneaky? It may even violate GDPR.
If you want delivery reports, just ask the user for it.
There is even a feature in the e-mail standard for it, that should be GDPR compliant, because the standard says, that e-mail software should ask the user before sending the report. If you want an unified UI across different mail programs, add a link "Please click here, to confirm that you received the message".
3
Mozilla may block the tracker in Gmail; see Content Blocking in Mozilla support. My guess is, you did not test Gmail under the configuration. Some stuff stops working as expected with Content Blockers, like reading articles online. I often get an [incorrect] message stating "Enable ads, please" when I really blocked a tracker. That is the price we pay to stop unscrupulous miscreants who attempt to abuse users.
– jww – 2019-06-28T06:10:20.99032Many client developers would consider the existence of such a mechanism as a serious bug and work to prevent it. – pjc50 – 2019-06-28T08:25:55.133
2Why do you want to do this? – Thorbjørn Ravn Andersen – 2019-06-28T14:27:09.783
1You can track if a mail is delivered (and when) e.g. if you own the SMTP. If the mail is a link to a page you own, well it can be tracked in the meaning that if you own the server you can know when that content was requested (and from which IP)... So indirectly you know it was read. – Hastur – 2019-06-28T15:29:13.473
22The "undetectably" part is the problem. You should not be attempting to trick or deceive your users. Such behavior is why software has to work so hard to prevent such malicious activity. If you really want to know if somebody read your email, "ask" them, whether manually or with automated tools. The fact you are trying to do it in a clandestine manner is what puts you in opposition to end users and the software that is designed for them. If you remove that requirement, you may have better options. – DKing – 2019-06-28T15:30:24.817
8The main motivation I can think of for wanting to do this undetectably is if someone is a spammer. They want to find out which emails on their mailing list are actually going to a recipient, and they want it to be automatic and undetectable because their interests are inimical to those of the recipient. – Ben Crowell – 2019-06-28T18:18:35.550
Why should a read confirmation from the recipients be illegal or intrusive? Many instant messaging applications show read status for the massages by default. If someone don't like this feature, he can disable it, and it will disabled for his sent messages also. Knowing that your sent mail is read or not, is the sender right. The recipient should reply, or even acknowledge that the mail has recieved: whether he want to answer or not. – Sd Hosseini – 2019-06-29T14:00:49.773
1
It is worth noting that Outlook supports email tracking natively (https://support.office.com/en-us/article/add-and-request-read-receipts-and-delivery-notifications-a34bf70a-4c2c-4461-b2a1-12e4a7a92141). It isn't "undetectable", but it does the advantage that when the reader opens the message, instead of being told about "unknown content" that could read their social media passwords and suck blood from their firstborn babies, they will be told that they got a read receipt and know exactly what it is.
– TheHansinator – 2019-06-30T07:04:46.5002@SdHosseini This simply isnt the case, if you instead wrote paper letters and mailed them to these users you would not know if they had recieved them (save for a service like requiring a signature, but even that isnt reliable as a secretary could sign for it but it could still get lost after that, plus the user would know you had done so.) there is no implicit right to know if an email reached its destination any more than there is for a letter. (Its an extra service, optional and the recipient knows you used it) – Vality – 2019-06-30T17:37:56.780
1Deeply unethical question. – iono – 2019-06-30T21:46:45.657