The theharvester script is a Python tool/script that searches for email addresses and domains using popular search engines. 

Example:

theharvester -d domain-name.com -l 500 -b google -h 

-d: is for the domain name
-l: in this case to limit the number of results to 100
-b: stands for the datasource, in this case we have chosen google but you have more options like (google,bing,bingapi,pgp,linkedin,google-profiles,people123,jigsaw,all)
-h: will use the SHODAN database to query the discovered hosts