[DanBC]

> Email providers are so scared of this that many officially require double-opt-in procedures in order to open an account

"Double opt in" is a flag-phrase that makes you sound like a spammer. If you want to avoid sounding like a spammer you should probably use the phrase "confirmed opt-in" instead.

Confirmed opt in is vital for unsolicited bulk email.

The example you give - unsolicited email, sent to a single person is not spam. As soon as it's sent to more than one person it's spam. Avoid being labeled as a spammer by asking people to opt in, and confirming their opt-in.