Back in 2016 gmail allowed emails to be sent programmatically. And the idea behind the project was to couple it with the email harvester and send thousands of emails to employers in an automated manner.
Emails were sent out using the PHPmailer library with a fixed time interval to avoid triggering any attention to the process.
I used GMAIL to send my emails to avoid them being marked as spam and I also limited the number of CC and BCC per email for the same reason.
The code used to select & mix emails randomly that belong to different countries (ex. se, fi, .uk etc). The code also selected random cover letters every time. The message content was formatted in HTML which contains google analytics pixel to check the percentage of the email opened.
The code (php script) was triggered in windows using a cron job which will create a log file about email sent and their count and the success or failure of the sending process.