Hello Tuan Nguyen,
Usually, the email logs are in one of these directories: /var/log/mail.log, /var/log/maillog, or /var/log/syslog.
Also, since you mentioned you're trying to configure it with Gmail, bear in mind that you will need to configure an App Password to make it work with that provider.
Please let me know if you have any questions.
Looking forward to your feedback.