That error means your mail server is rejecting the connection because it doesn't recognize SubsiMail's server as authorized to relay mail through it — this is usually an IP allowlist or authentication setting on your mail server's side rather than anything in SubsiMail. Double check: are you providing a username/password (not just host/port) in Settings > Sending? And if your mail server restricts relaying to specific IP ranges, you may need your IT team to allowlist the IP our server is connecting from.