Right Inbox - Schedule Emails in Gmail To Be Sent Later
EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.
Seventh Sense - Seventh Sense helps marketers and sales professionals reach the people that make them successful using artificial intelligence powered email.
Email Studio for Gmail - Send personalized emails, schedule emails directly in Gmail
RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.
If No Reply - Simple email automation that works for Gmail