Sortd - Rated the #1 App for Gmail
Right Inbox - Schedule Emails in Gmail To Be Sent Later
KanbanMail - A Kanban board for your emails.
Recurrr - Send recurring emails
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
FollowUpThen - The simplest way to schedule an email reminder