Attempt to use the name of the domain directly when mail exchange lookup fails.
| Parameters | failure | The reason for the lookup failure. (type: Failure) |
| domain | The domain name. (type: bytes) |
| Returns | An MX record for the domain or a failure if the fallback to domain option is not in effect and an error, other than not finding an MX record, occurred during lookup. (type: Record_MX or Failure) |
| Raises | IOError | When no MX record could be found and the fallback to domain option is not in effect. |
| DNSLookupError | When no MX record could be found and the fallback to domain option is in effect but no address for the domain could be found. |