@Lopro - Did you ever get your setup working with Oracle mail?
Running discourse doctor is giving:
==================== MAIL TEST ==================== For a robust test, get an address from http://www.mail-tester.com/ Or just send a test message to yourself. Email address for mail test? ('n' to skip) [<snip>@gmail.com]: firstname.lastname@example.org Sending mail to email@example.com. . . Testing sending to firstname.lastname@example.org using smtp.email.uk-london-1.oci.oraclecloud.com:587. SMTP server connection successful. Sending to email@example.com. . . Sending mail failed. Replacing: SMTP_PASSWORD Replacing: LETSENCRYPT_ACCOUNT_EMAIL Replacing: DEVELOPER_EMAILS Replacing: DISCOURSE_DB_PASSWORD Replacing: Sending mail to ==================== DONE! ====================