I know Magento2 Enterprise Edition has this functionality already built into it. At the moment I am using a basic SMTP module to send all the transactional emails on my website, however, the process of sending emails to SMTP is the thing that delays the most my website. Do you know any module to do so, or can you suggest how can I develop one?
I prefer to use postfix queue system, it works better than magento built-in queue system. Just install postfix on your local machine, setup a smarthost as relay. you can relay to amazon SES, SendGrid, or any remote SMTP service. Then you need to point your magento2 to send email to localhost:25. I don't remember what method the builtin magento2 system is using. I had a custom smtp module that previously was built to send email to remote smtp but decided to send through postfix instead. Trust me, it's easy to setup.
If the default magento doesn't send to localhost:25, you can try this https://webkul.com/blog/magento-2-send-mail-using-your-smtp-detail/ change the parameter to localhost port 25 and remove tls.