[tyleo]

Most mailboxes have a little plastic flag on a hinge you flip up to mark that you have outgoing mail. The process is is generally:

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...

[qwertfisch]

Wow, I always had the impression the red flag is set by the mail carrier to indicate for the house owner (particularly in rural areas) that new mail is inside the box. (Hence it would save the owner a trip for checking.)

But this way around it makes a lot more sense.

[lesuorac]

Tbh, as long as you make the mail obviously not placed by them they'll take it. (as in don't need to use a flag).

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.

[al_borland]

Some mailboxes have an indicator that mail has arrived, but they aren't the norm. The indicator is raised by the action of opening the door, so the mail carrier doesn't have to do anything. It is on the person getting the mail to reset the indicator. This indicator is separate from the flag that signals the carrier that there is mail to pick up.