A catch-all mailbox is one that collects all the emails sent to invalid email addresses. For instance, if your address is firstname.lastname@example.org, but somebody mistypes it and sends an email to email@example.com, it will still be received in the catch-all mailbox that you have created. Just one mailbox per domain can have this feature enabled and in this way you will never miss emails because the other side has an out-of-date email address or has made a typo. The downside of setting up a catch-all mailbox is that you may begin receiving a lot of unsolicited email messages, but this issue can be rectified using an anti-spam protection solution. It is also vital to remember that a mailbox with an active catch-all functionality cannot redirect incoming messages to a 3rd-party address.