If you leave mail in your mailbox the mail will continue to pile up and eventually no more mail will fit into the mailbox.
If the USPS is noticing that you're not collecting your mail within a week most often then they will remove your mail and take it back to the post office.
If your mailbox is full, the mail carrier will also leave a "We ReDeliver for You" form (PS Form 3849) in your mailbox.
This form will also include any instructions on how to schedule a re-delivery of your mail or pick up the mail from the local post office.
If your mailbox is consistently full then the mail carrier might stop delivering mail to your address and may even mark your house as being vacant.
The post office will also hold your mail for up to 10 days and after 10 days the mail will then be returned to the sender.
If you want to send any mail from your mailbox be sure to put the flag up which indicates that you have mail to be picked up.
You can also drop the outgoing mail in a blue collection box or at the post office.
Always be sure to collect your mail from your curbside mailbox often and even if you get mail at the post office at a post office box, be sure to collect your mail at least every few days as well.