For the sandbox environment, the SMTP and API credentials are maintained separately.
In order to locate your SMTP credentials for Sandbox environment of your Netcore email api account, you can follow below steps,
1. Login to your Netcore account, URL: https://email.netcorecloud.com/
2. Go to settings => sandbox
Here you can find the SMTP credentials of the sandbox environment for your Netcore account under "SMTP Relay" section.