Ok, looking at what is sent back in the return receipt, there are a two attachments following the body text (disposition mime header and the original header). I don't see anything in them that I don't already know, e.g., my address, the recipient's address. There are some routers the message went through in the original header. Not sure what is private about this. Also, I don't see a record of the sent return receipt being saved by the sender of the receipt anywhere. So nothing to see there other than the originally received message.
Bug 257309 Comment 11 Edit History
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Ok, looking at what is sent back in the return receipt, there are a two attachments following the body text (disposition mime header and the original header). I don't see anything in them that I don't already know, e.g., my address, the recipient's address. There are some routers the message went through in the original header. Not sure what is private about this. You can see the same thing if they reply the message. Also, I don't see a record of the sent return receipt being saved by the sender of the receipt anywhere. So nothing to see there other than the originally received message.