postbox (outgoing mail)
A physical receptacle into which outgoing mail items may be deposited for collection and subsequent delivery by the postal service. Also known as collection box, pillar box*, drop box, etc. (Wikipedia)
Examples: post #888398, post #870912
For tagging purposes, distinct from mailbox (incoming mail), which is for incoming mail.
*However, pillarbox is used here to refer to the image composition technique.
See also
The following tags are aliased to this tag: mail_collection_box and postbox (learn more).