Do I have to label my marketing mail as an advertisement?

I saw somebody ask recently how best to label their email marketing messages as an advertisement. Their intent was to comply with CAN-SPAM, the US federal anti-spam law. Though I am not a lawyer and this is not legal advice, allow me to clarify it with my layman's pants on, since CAN-SPAM misunderstandings and myths abound and this is an easy one to help folks understand.

According to CAN-SPAM, you do NOT have to label your email as an advertisement, if you have "prior affirmative consent." Meaning, if all your email is opt-in, you only send marketing messages to people who have explicitly signed up to receive such email from you, then you're good. You don't need to label the mail as advertising.

But don't take my word from it -- here's the relevant detail, straight from the fine folks at the Federal Trade Commission (FTC):

"If recipients have given their prior affirmative consent to get messages from you, you're exempt from the requirement of identifying the message as an ad or solicitation."

(It's an excerpt from a longer answer, tackling a broader question -- see for yourself, if curious.)

And since just about every email service provider or email marketing platform requires all clients to send mail only to opted-in recipients, if you're already complying with their rules, you're probably going to be covered as far as affirmative consent goes, right?

All of this adds up to yet another reason why you should never purchase an email list -- that is not the path to opt-in, and thus, not the path to deliverability success.