"Remind me about this later" is one of the most common small frustrations in email — and three fairly different products have grown up around solving it. They're not interchangeable; each one assumes a different starting point.
FollowUpThen has no app to install. You CC or send to an address like 3days@followupthen.com and it replies with a reminder at that time. Because it's just email, it works identically in Gmail, Outlook, or anything else — no browser extension, no client lock-in. The Basic plan is free; Personal, at $4.99/month, adds notes attached to the reminder. Limitation: it only knows about email. It won't track a reminder that has nothing to do with your inbox.
Boomerang is a browser extension that lives inside Gmail specifically, adding send-later scheduling, "remind me if no one replies," and messages that boomerang back into your inbox at a chosen time. If your whole workflow is Gmail in a browser tab, the integration is genuinely tighter than an email-based tool can offer. Outside Gmail, it doesn't exist for you.
Reminders.md doesn't assume the reminder originated in email at all. You can create one by talking to Claude Desktop mid-conversation, from a terminal with the rem CLI, from the web dashboard, or via the API — the trigger doesn't have to be an email you're looking at right now. Recurrence uses real RRULE patterns rather than a simple repeat interval, and every reminder email that goes out includes one-tap complete and snooze links, so acting on it doesn't require opening the app.
If the reminder is always about an email and you want zero setup, FollowUpThen is hard to beat. If you live entirely in Gmail in a browser, Boomerang's native integration is worth it. If your reminders come from all over — a Claude conversation, a terminal command, something that has nothing to do with your inbox — a tool that isn't email-shaped in the first place fits better.