If there is no index, no key, no trigger, and that you have a low isolation level such as `Read uncommitted`, I really don't see what the database could do more than an insert.
It's a front-end application around a SMTP mail server.
https://datacadamia.com/marketing/email/server
You take your favorite language and you build your own.
That's hard ... ip control (they may be blacklisted), high availability, bounce and other features requires a lot of work.
The easy way is to create your application around a transactional email service. You pay for the email but not for the application :)