If you leave mail in your mailbox and it's a curbside mailbox or a house mailbox then the mail could be stolen easily.
Also eventually as your mail piles up the mail carrier will likely remove your mail after a week or so and then put a notice in your mailbox to retrieve the mail from the post office.
So make sure you get your mail out of the mailbox daily if possible.
You can also mail things from your mailbox too.
If you don't have a flag on your mailbox then you can just put the mail into the mailbox so that the stamp is seen and shows the mail carrier that it has not been postmarked.
Or if your mailbox is on the outside of your house you could put a second mailbox on the house and have one for outgoing mail and the other for incoming mail.
My neighbor did this and he put incoming mail on one mailbox and outgoing mail on the other mailbox and told the mail carrier that he knew what it was for.
Then he could mail things from his outgoing mailbox and get his incoming mail from the other box.
Yes you can mail things by putting them in your curbside mailbox.
Just put the flag up when you put your outgoing mail in the mailbox and when the mail carrier sees the flag up they will know that you have outgoing mail and retrieve the mail.
Then the mail carrier will put the flag back down for you.
If you mail something from your mailbox then you must put the flag up to alert the mail carrier to outgoing mail.
Otherwise if they don't see the flag up they won't realize you have outgoing mail in the mailbox and not retrieve it.
The mail carrier will put the flag back down after they retrieve your outgoing mail and if they have other mail for you then they will place your mail in your mailbox.
You do put the flag up on the mailbox if you're sending outgoing mail.
The red flag on a curbside mailbox is for sending outgoing mail.
When you have outgoing mail to send you put the mail in the curbside mailbox and then raise the flag.
The red flag on the curbside mailbox signals to the mail carrier that you have outgoing mail in the mailbox.
Then the mail carrier will stop at your box and remove the outgoing mail to send it off and then they will lower the red flag.
So even if you have no mail coming to you directly on that day you can send outgoing mail from your mailbox and the mail carrier will pick the mail up.
Just make sure to raise the red flag so they know the mail is in the mailbox for outgoing mail.
I've mailed letters through my mailbox before and the mail carrier always lowers the flag down after they get the mail out.