NudgeMail - Write or Forward any email to NudgeMail.
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
FollowUpThen - The simplest way to schedule an email reminder
Rebump - Automatic follow up emails. Keeps on bumping until you get a reply.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Tomba.io - Tomba is the leading solution to find and verify professional email addresses. Start using Tomba and connect with the people that matter for your business.