Justin is right. Webcrawler scripts are very smart these days. However, if you have access to a bot net then there is a much quicker way to harvest valid email addresses. Just query the target email server directly by pretending to be a mail server with a message to be delivered.
The handshake procedure between the servers go something like this in plain english.
Sending server: I have a message for
joe@targetdomian.com
Receiving server: OK, I have a email account by that address, send the message. In this case the bot logs the email address as valid and drops the connection.
or
Receiving server: Sorry, that is not a valid user and drops the connection.
With even a tiny botnet of a few hundred thousand bots, you can build a list of valid email addresses at a domain in a few hours.
This works because the bot net is using different IP addresses to make the connections and a connection dropping prematurely is not uncommon. Now if you tried this from your home Internet connection, the incoming server would flag your IP address and block it after a certain number of invalid user attempts.
Of course, getting the spam past the spam filter is a whole different problem for the spammer but large list of valid users on a server can fetch a good amount of money on the dark web.