SendPulse for Slack lets you send email and webpush campaigns from Slack.

Add new subscribers and edit your mailing lists, create and send email and web push messages directly from Slack. Get stats about your email campaigns. Receive notifications to your Slack channel or request them from the bot.

This SendPulse Slack Bot if absolutely free. SendPulse e-mail service is also free for sending up to 15,000 emails every month.