That's very interesting. How do they know what's outgoing? Do they have to check the address of each letter, or is there like a separate compartment for outgoing mail?
1. Remove all mail from the mailbox
2. Place outgoing mail in the mailbox
3. Raise the outgoing mail flag
4. Carrier will empty the box and file outgoing mail during the next delivery before putting new mail in the box
Edit: here is a good example of a mailbox with that red flag https://www.amazon.com/Step2-541200-MailMaster-Mailbox-Black...
But this way around it makes a lot more sense.
Like they always shove (or drop) it into the mailbox so when there's letters that are obstructing the box from closing properly they know they didn't place them there so it's outgoing and they grab it.