Thank you very much Zach!
We do use SendGrid as the default SMTP server in the Kentico setting with batch size 3000 and archive email days 365, but it's too slow. It says the emails are distributed one by one to the available server. From what I read we need to use SendGrid API to speed up the process. We don't want to send one by one. Any suggestions how we should configure inside Kentico to speed up the emails? Adding more SMTP servers? Network guys don't want to use pickup directory option.