Before implementing my own SMTP module for Magento 2 I have tried a lot of alternatives.
1. Mageplaza SMTP Extension
It will spam you:
- via built-in Magento backend notifications
- via email (the extension requires email registration)
2. MagePal Smtp App
It just does not work for me: I have got a lot of PHP errors.
It looks like the extension is built for an outdated Magento 2 versions despite the last commit was just a day ago.
Maybe MagePal developers do not test their extension with all Magento 2 versions?