MailClark - The Slack bot for external communications

Right Inbox - Schedule Emails in Gmail To Be Sent Later

iDoneThis - iDoneThis The fun, easy way to get stuff done. Every day we’ll email your team to ask, “What’d you get done today?” Just reply to our email. The next day, everyone gets an email with the team's accomplishments

Recurrr - Send recurring emails

PigeonBot - Send emails via Slack with the /email command

FollowUpThen - The simplest way to schedule an email reminder