I fixed this by resetting the admin config file as follows:
Log in to the Linux server
Stop the mail service:
sudo service smartermail stop
Edit the file:
sudo nano /etc/smartermail/administrators.json
Find the admin entry you want to reset (usually "username": "admin":).
Replace the entire password hash value with the official default hash:
"1000:7Zf5bDhx+pB+3pSt9bFzbnnn9uiZi25P:+esCLUvFa/lw8c1+UMk+guGeHFfQ7Asc"
This hash corresponds to the default password admin.
Save and exit.
Restart the mail service:
sudo service smartermail start
Login:
Username: your admin username (e.g., admin)
Password: admin
Immediately change the password to something much stronger.