Idea Summary
Currently third party SMTP host setups are not supported in APEX on Autonomous Database. We can configure the internal customer SMTP hosts with or without Authentications in APEX Instance Configuration, but it does not clears the queue when we use APEX_MAIL package to send an email.
Use Case
Sometimes customer wants APEX cloud applications should be able to send emails to it's Internal customer and they should be Internal, not marked as External emails. Sometimes to achieve that they need us to relay the mails through the Internal SMTP servers. Which is probably is not supported in APEX on cloud Autonomous Database. It only supports relay through Oracle provided SMTP servers.