It may depend on the content of your email, but if you're referring to CAN-SPAM, "relationship" emails are not covered by it. As always though, if you are concerned with the legality of something, you'd need to talk to a lawyer with relevant experience. I can't say that I've read through the specifics to tell you the specific requirements, though Wikipedia just refers to an operable unsubscribe link relating to what you're referring to. (There are other content-elements which you may not be meeting, such as physical addresses.)