Various vendors have defined so called “mail-in” APIs, which trigger certain automatic behaviour based on commands and keywords written into an email.
- Mailinglist software (“Reply with SUBSCRIBE in the body”)
- Customer service ticket systems
Mail-in APIs can be considered an ad-hoc, vendor-specific approach to structured email.
Mail-In APIs are simple to use and fits well into existing email usage workflows and client software.
Mail-In APIs are usually not interoperable, but work just for a very particular use case. Users and tools involved need to obtain prior knowledge about the command syntax.