Under GDPR rules you need explicit permission from an EU resident to track any emails in which they are the recipient. So this is essentially the same as adding a read receipt to a message; the user does not have to give permission for you to track the message, and they don’t need send a read receipt. In most cases, tracking with permission and read-receipts will only be done for a very small percentage of your messages, so as a business tool it is useless. Some servers do not even send read-receipts even if the recipient gives consent.
If your other applications like Mailbird are allowing you to track emails without the recipient’s consent, then that is illegal. As a business, do you really want to be doing that?
For legal tracking, it may be something that can be managed in a campaign system like MailChimp, but how do you propose to implement that in an email client?