When done right, it works.

It's the hubspot/marketo approach of slamming inboxes with daily/weekly, non-specific garbage that needs to go. And buying lists or using lists from conferences - fuck that, it's just spam.

Following up with someone who explicitly gave you permission to send them emails after signing up for your service is reasonable and expected IMO.

It's the only channel that isn't owned by a walled-garden, so investing in it and treating it with respect is essential since it's the only way to communicate to customers and potential-customers without a middle-man extracting rent.