Where can I find the PIN for external users
If you forgot the PIN that is needed for external users wanting to access a temporary mailbox, do the following: Go to your e-mails, open the Sent objects folder, and select the first e-mail that was previously sent to the temporary mailbox. Open the context menu of that e-mail and find the menu option that says Check assigned PIN to display the number.
Alternatively you may have a look at the source of the first e-mail that was previously sent to the temporary mailbox. You will see the followig line: