[PeterisP]

Opt-out lists are a big issue.

A checkbox hidden somewhere that's enabled by default doesn't mean that "I have subscribed" to the mailing list, both in the common sense and also is prohibited in many legal jurisdictions (not USA, AFAIK), leaving a default checkbox simply doesn't count as obtaining consent, it is a well known 'dark UI pattern' that even the consumer protection laws have understood and explicitly implemented.

Such messages are just another kind of spam, and the right thing to prevent this, naturally, is to block the sender as a spammer. Underneath there is somewhat proper a mailing list that supports unsubscription, so an unsubscribe message can and should also be sent, but it doesn't change the fact that all the subscribers were added without their informed consent - if you built a system with opt-in by default, then you yourself built a system where it is impossible to say that you actually want to subscribe, the only provided checkbox then represents a choice between the default (no informed consent) and an explicit refusal of consent.

[tajen]

I'm the author of a product and in some situation I have subscribed former users to a one-off email. It's gray area since it was to notify them about a major release/change, it was written in my T&C and sign up form, but honestly it would feel legitimate if they marked my email as undesired/spam.

Out of 900 recipients, 1 marked it as spam, 18 unsubscribed. I believe people are very tolerant and they could use "spam & blame" a lot more.

[linkregister]

I think your situation is different; you're not subscribing them to a newsletter. Your communication is reasonable for a typical business relationship. (and is protected under CAN-SPAM)