Problem:
Cannot send test email notification in toolset forms

Solution:
The email goes to spam/junk due to any of the listed reason with the following link:
=> https://popupsmart.com/blog/why-emails-go-to-spam-instead-of-inbox

You should set your email to "Not a Spam" or "Not a Junk" and from next time it should be in your inbox.

If you want to set email headers while sending email, you can use the Toolset form hook: "cred_mail_header"

You can find the proposed solution in this case with the following reply:
=> https://toolset.com/forums/topic/cannot-send-test-email-notification-in-toolset-forms/#post-1770251

Relevant Documentation:
=> https://toolset.com/documentation/programmer-reference/cred-api/#cred_mail_header