Signup Crew checks recipients before sending. A Volunteer may be excluded when the current email is missing or invalid, the profile is archived, the person unsubscribed from broad Organization updates, the address is suppressed after a delivery problem, or multiple profiles share one destination.
Unsubscribing stops broad Organization updates. Appropriate Event-operational messages may still be sent. Organizers cannot override suppression.
“Accepted by the email provider” means the provider accepted the request; it does not guarantee inbox delivery. Ask the recipient to check Spam or Junk and confirm the address. If the problem continues, contact Support with the Event, recipient address, approximate send time, and visible status—never a password.