Mail is an item that's being carried by a courier during delivery.
Delivered mail will show a sender address in item's tooltip.
Mailable
To apply an Address Tag to the item or insert the item into
Mailbox sending slot, it needs to have a #envelope:mailable tag.
Mailable items in Envelope:
Delivery Log
Delivery log has information about sender and recipient addresses, as well as other information related to the delivery: payback status, return reason, etc.
When a mail was delivered, holding Shift will show a delivery log of this item, instead of a sender's address.
Info
Delivery Log is also shown in the inbox, when hovered over an address.
Delivery Log tooltip