ServiceNow had (still probably has) something called inbound actions. This is people clicking on mailto: hyperlinks in servicenow notification emails to do stu

ServiceNow had (still probably has) something called inbound actions. This is people clicking on mailto: hyperlinks in servicenow notification emails to do stuff like approve/reject a service request or reopen an incident ticket. You can go beyond this functionality and build any workflow to be triggered after consuming an inbound email sent to client@servicenow.com I am sure there are a number of email based integrations built using this especially when the upstream application didn't speak API but could do email.