The dnsmap command scans a domain for common subdomains e.g. smtp.domain.org. More information: https://github.com/resurrecting-open-source-projects/dnsmap.
dnsmap {{example.com}}
dnsmap {{example.com}} -w {{path/to/wordlist.txt}}
dnsmap {{example.com}} -c {{path/to/file.csv}}
dnsmap {{example.com}} -i {{123.45.67.89,98.76.54.32}}