IMO, Email systems originally have been designed that way to facilitate traceability. When webmail came, it's natural that they followed this practice.
Facebook is a closed system. When you send a message to someone on Facebook, you presume that you are adding that message within Facebook. The notification email that Facebook sends to the recipient is just that, a notification generated by the Facebook system. You, the sender of the message is not part of this notification system, though you your action triggered the notification. There is no reason why the sender's IP should be exposed in this flow under normal circumstances.
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IMO, Email systems originally have been designed that way to facilitate traceability. When webmail came, it's natural that they followed this practice.
Facebook is a closed system. When you send a message to someone on Facebook, you presume that you are adding that message within Facebook. The notification email that Facebook sends to the recipient is just that, a notification generated by the Facebook system. You, the sender of the message is not part of this notification system, though you your action triggered the notification. There is no reason why the sender's IP should be exposed in this flow under normal circumstances.