Comment by traceroute66

> Instead, send them a politely worded one-time announcement with an invitation to subscribe.

NO. DO NOT DO THAT !

That is terrible advice and it is against the law.

Opt-in has to be done without inducement and of a person's own volition.

Sending a mail to someone saying "pretty please sign up" is not valid opt-in. It is spamming a bunch of people hoping they will opt-in. It does not matter if you got their mail another way (e.g. if they purchased a product, you can't then spam them trying to get them to opt-in for your mailing list).

One of the fundamental reasons the opt-in law exists is to stop people doing the shit you suggest and ensure that lists are correctly built in a clean manner.