Secondary Mail Server
When an email is sent to a recipient the email server looks to see which server handles mail for the domain using the DNS MX records. MX records specify which server(s) will receive mail for a given domain. Each MX record is given a priority (the MX level), the lowest being the primary mail server. The primary mail server is responsible for delivering the mail to the appropriate user mailbox.
To monitor a mail delivery scheme with more than one SMTP server you can enter a secondary mail server to be used in case of a failure of the primary server. To activate this feature add the hostname or the IP address of the secondary MX server at the "Second SMTP host" field.
Mail delivery is first attempted to be delivered to the primary mail server (the one defined in the SMTP host field). If that server is not reachable then the mail will be attempted to be delivered to the second mail server. If the second mail server can not be reached then an error will be recorded and alerts sent accordingly.
This article applies to Email round-trip transaction monitoring only.