utl_smtp.starttls gets "ORA-29024: Certificate validation failure" when attempting to send email

My platform: Oracle 19.12.0.0.0 Enterprise Ed. on RHEL 7. No RAC, no ASM, no PDB

The mail service I use is smtp.mailgun.org

The test script I use is similar to the one in "Basics on How to use UTL_SMTP (Doc ID 106513.1)".

Sending email using that script is successful ?

Next step: Use TLS for encryption. I have,

1. Extracted ssl certificate using,

openssl s_client -host smtp.mailgun.org -port 587 -prexit -showcerts -crlf -starttls smtp

2. Created Oracle wallet and added the certificate into the wallet.

3. Changed the script in 106513.1;

  (a) added wallet path and password to objConnection := utl_smtp.open_connection