If you have a curbside mailbox and leave your mail in the mailbox for longer than a week or two and the mailbox gets too full then the mail carrier will usually remove the mail in your mailbox, leave a notice in your mailbox and take the mail back to the post office.
If they don't hear back from you within a week or so then the USPS will send the mail back to the sender.
So you should make sure to remove your mail from your mailbox often to avoid the mail piling up and prevent future mail deliveries.
Also leaving your mail in your mailbox at the curbside also makes it easy for someone to steal your mail and it alerts someone that you're likely not around.
Then someone can steal your mail, do identity theft, and even break into your home.
If you're gonna be away from home longer than a few weeks then contact the USPS and have them temporarily hold your mail or temporarily forward your mail to your new location.
If your mail is in a post office box and you don't pick the mail up then they may pull the mail from your post office box as well and put a notice in your box that they are holding the mail or they may call you and alert you to the mail needing picked up.
Then if no answer they send the mail back to the sender.