Do return receipts work at all when you send email from Thunderbird on Windows to someone who uses Apple Mail? In the test I ran, the answer is no. Here is the test I ran:
- I sent email from Thunderbird on Windows to someone's personal address and they read it on Apple Mail on a Mac. I got no receipt. They did not see any notification about "sender asked to be notified when you read the message...". They have never made any changes to the settings that might block receipts.
- I sent email from Thunderbird on Windows to someone's work address and they read on Outlook on Windows. I got a receipt. They did not see any notification about "sender asked to be notified when you read the message...".