Drip email means to deliver a series of emails, "dripping" into the inbox, so called because of drip irrigation.
In the 1950's, "drip" was slang for someone who is very uncool. but over the years, using "drip" fell out of style.
Then around 2018, the word "drip" had a renaissance thanks to hip-hop. It meant the complete opposite "uncool". Now "drip" means to have a sexy or cool style, wearing expensive jewelry or clothes, to have swagger.
It is hard to pinpoint who started the revival but many take credit.