Boomerang
8.1#2 in sales email tracking softwareFrom $4.98 a monthFree planAndroidiOSWebAPI
VisitBoomerang suits Gmail users who want to schedule messages, pause incoming mail, and manage follow-ups. Its team scheduling tools may suit organizations coordinating meetings across calendars.
Mailproof suits Gmail users who want open and link tracking, scheduled sending, and a choice between daily summaries and real-time alerts.