Followup.cc might be a bit more popular than infrequent. We know about 1 link to it since March 2021 and only 1 link to infrequent. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
I signed up for https://infrequent.app/ when it was a Show HN just to try it out and it actually worked (probably about as well as a "non-busy" TODO in my calendar app with an alert). Enough to remind me to call 1 or 2 people that I like to talk to but often leave for too long, anyway. - Source: Hacker News / over 1 year ago
I'll update this if I find more. For now, this seems to be a partial solution. It's a Gmail add on (except I don't use Gmail) that adds reminders. The options when setting reminders include cancelling it if someone replies, and to remind everyone in the thread. If you combine those two, it seems like — if you use Gmail's web client and have $23/month you're dying to get rid of — a decent way to add the feature. Source: over 1 year ago
Dex - One place for your relationships — impress with thoughtfulness
FollowUpThen - The simplest way to schedule an email reminder
FollowUp - Upgrade your inbox
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Keep In touch - Keep in touch with thousands of people
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.