Use Send API

For example: https://github.com/danielbachhuber/mandrill-wp-mail
mailgun plugin
sendgrid plugin etc.

Please consider this.

The plugin ‘mailjet for wordpress’ contains API libraries (in sub-folder api/ and elsewhere) to fire apiv3 actions. It is possible to create your own plugin beside this one to define/modify newsletter (and include a complex content), attach this to a contactslistid and fire a campaign at a scheduled time. Be aware that some features needs class extends or small modifications (committed to mailjet developers…)
M.
Available docs : https://dev.mailjet.com/email-api/v3/apikey/

I was just talking about switching from SMTP to API based email sending (single email).

Is there such a thing in the API? I mean single email sending.

Please consider implementing API based email sending in wp_mail().
Thank you.