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