The process of extracting different emails(which are not useful) through any method is known as email extraction. The main purpose of email extraction is to reduce spamming through emails.
There are various methods through which the process can be done are extracting the emails manually and the other one is extracting the emails through email extractor.

Extracting email addresses from different sites
You can extract email addresses from through different websites directly by typing in a search engine if you are targeting any specific website or searching for few websites, but if you are targeting more than hundreds of sites than you must try tools to extract the emails from different sites.
Email Extractor
Email Extractor is a powerful Extracting Emails extension software. It automatically fetches valid email IDs from the web page, you can create a copy of particular email IDs you need.
-AutoVisit: it built a queue of URL that you want to visit automatically
-Automation: discover all email ID’s ending with any specific name
-extract email address from webpage automatically
-It helps you download stored email IDs
-share your premium subscription with your colleagues
-Extracts cloaked emails
-Filter for duplicate email ID, so that you can get the unique ones
-export option to the export list of your extracted emails to a text file
-extract emails from a local HTML document
-extract emails from text files
-removes duplicate email addresses
These features may vary through different software as they all are not the same and their functions are based as per requirement. There a huge range of scrapping tools from free to paid. Email extractor varies from the type of information they gather: different ranging from emails and phone numbers.Online email extractors are very useful to reduce the spamming.