The SMTP credentials for Production Instance should be shared by Client. Which you can then configure in "Day CQ Mail Service".
Fo test accounts, you can create Google credentials. But, these are not for production use.
Hello @Prashanth_02a -
To set up SMTP for sending email notifications from your AEM production instance, you will need the SMTP server details & need to configure those in AEM :
Once you have configured the SMTP settings, you can use the AEM EmailService API or the Email Notification step in workflows to send email notifications from your AEM instance.
Adding onto the steps mentioned by @Tanika02 on how to configure SMTP on AEM, make sure the SMTP server is reachable from AMS/AEM as CS Author/Publish servers.
If the SMTP server in question is protected behind the organization's firewall, make sure it's reachable by allowlisting via IP/IP ranges.
You may use "telnet SMTPSERVERNAME 25" command by ssh'ing into Author/Publish server to test the reachability.