Magento 2 SMTP Email Extension by Webkul helps store owners set up an SMTP server, which transmits messages in codes or numbers. With the help of this extension, email messages are delivered directly and automatically to the selected customers in their inboxes. This prevents email messages from going to the spam box.
To know more visit- https://store.webkul.com/magento2-smtp-extension.html
So, like, you're saying that self-hosted MTAs never have problems with messages landing in spam? Sure...