Can My Ex-Employer Deny My Relieving and Experience Letters After Sending an Email?

I received a relieving email. Through this email, I have already joined another company. Now, my ex-employer is asking for the relieving letter and experience letter, which I will provide after completing pending work.

The relieving email only mentioned one line stating that I am relieved from my roles as of this date.

1. Can they deny me the relieving and experience letters?
2. Is the relieving email valid?

If the email is from the competent authority of the HR department, that is valid. On that basis, they cannot deny you an experience letter. However, if some person not competent to relieve you without any formal order from the HR, that may not work.