From their own pricing pages, checked on 11 November 2026: Mixmax has a free tier for 50 recipients a month and tracks opens, clicks and replies, with Gmail named as the client; Saleshandy has no permanent free plan but a 7-day trial and tracks opens, clicks and replies; GMass works in Gmail, has no free plan (it can be tried from the Chrome Web Store) and reports opens, clicks and replies.
If you run cold outreach with personal fields and automatic follow-ups, those tools do more. If you write to people one by one and want to know which signals were likely machines, HawkOpen is the narrower, more careful choice. Its Safe Follow-up on Business writes a draft but never sends automatically.