What is Backup MX?
Backup MX (also known as SMTP Backup or Secondary SMTP) is an email service that sits at a lower priority then your primary mail server.
Backup MX is essentially a SMTP Server that will accept email for your domain if your primary SMTP mail server fails or goes offline.
When your server is offline the Backup MX SMTP server will accept the email and store it until the primary server (or alternative SMTP server) is back online and ready to accept emails again. Then those emails on the Backup MX server will be delivered to the main server.