Send it in three weeks - unless they get back to you first

Draft an email with the assistant and tell it when to go: “in 3 weeks”, “next Tuesday”, “tomorrow”. It works out the date, shows it to you, and asks before queuing anything - because the send happens when nobody is watching. The useful part is the condition. “Follow up in three weeks unless they reply first” is a real setting, not a reminder to check. If they write back before then, the follow-up doesn’t go, and it tells you why it didn’t - “they replied on the 3rd” - rather than silently doing nothing, which is indistinguishable from a bug. You can also make it “unless anything happens on that record at all”, and schedule to several people in one go, then cancel the whole batch with one instruction. Two things it deliberately won’t do. It won’t treat a colleague’s email about the same customer as “they replied” - you still owe them the follow-up. And it re-checks, at the moment of sending, that you’re still allowed to send from that address and that the recipient hasn’t unsubscribed in the meantime. Three weeks is long enough for both of those to change.