Do I need a mailbox at the ISP?
If you get a static IP address at your ISP, you can chose ETRN mail server mode. This option does not require you to set up a specific mailbox at your ISP. Your ISP only needs to add a MX record for your mail server.
If you get a dynamic IP address for your office LAN from your ISP, you can chose mail server mode as multidrop. With this option, you need one mailbox and your mail domain name at your ISP, eg. mailbox name: companymailbox, mail domain name: yourcompany.com.