mx_failures_fallback_to_a — configure the maximum number of times an MX lookup will be attempted
mx_failures_fallback_to_a = 3
The number of times an MX lookup will be attempted before attempting an A lookup. The default value is "3".
If the value is set to -1, the first lookup will be an MX lookup, and any subsequent lookups will be with A records.
mx_failures_fallback_to_a is valid in the domain and global scopes.