Mailboxes with the red sticker are Mail boxes German Post.
In addition to being delivered directly to post offices, franked Deutsche Post letters can also be deposited in additional letterboxes, which are regularly emptied by the post office. Letters are collected from some mailboxes every working day, from others only once a week. The exact emptying days and times can usually be found on a sign on the mailbox, as well as on the red round sticker.
This means in particular that the mailbox every day, including public holidays and Sundays is empty. Daily collections can be particularly important for senders who want to send letters as quickly as possible at short notice or with a fixed arrival date.