For commercial email to US recipients, yes — that's an actual legal requirement under the CAN-SPAM Act, not just a best practice. You need a valid physical postal address (a registered P.O. box is acceptable) in every commercial email. It's easy to add once under your default email footer/template so it's automatically included on everything going forward, rather than needing to remember it per campaign. Worth noting CAN-SPAM has a few other requirements too — a working unsubscribe mechanism honored within 10 business days, and no misleading subject lines or header information — all of which SubsiMail's default sending flow handles, but the physical address is the one piece you do need to add yourself.