Cold email is slang for an email that is sent to an email address that has not opted in to receive emails from you. "Cold email" is a phrase generally used in a pejorative sense, since response rates (and engagement overall) are much worse for cold emails than for regular emails. (Think about it from the perspective of someone emailing you: are you more likely to open an email from a business that you signed up to learn more about, or from someone you've never heard of?)
In general, Buttondown does not support cold email. We require all subscribers to double opt into their newsletter, and we reserve the right to offboard newsletters which appear to be skirting our terms of service in order to send cold emails. Additionally, the CAN-SPAM act prohibits many forms of cold email.
If you're interested in sharing more about your use case and checking to see whether or not Buttondown supports it, please contact us.