Email scraping, also known as email harvesting, is when one obtains individual email addresses or email address lists using different methods. Usually, these email addresses are then used for email blasts or spam.
Trading email lists with parties who have them to offer is one of the most common methods to acquire new email addresses.
Bots and crawlers are used often as well. Email addresses are made up of two parts—a username and a domain name, separated by @. This specific format makes it not that difficult to create bots and scrape the internet looking for relevant email addresses.
Even if bots pick up wrong email IDs, a spammer can remove the address from their list after sending the first message and realizing that it bounced.