AirHelp.com helps passengers get compensation for delayed flights. We’ve combined deterministic programming (RegExp, XPaths, etc.) with some fancy NLP stuff to build travel email parsers that perform equally well as Google’s proprietary parsers. Now we’re thinking about allowing anyone to use our parsing API to power their own projects, for free. In exchange we'd like to inform email recipients, that have been delayed, that they deserve compensation.
Would you use it?
For starters, it’s now a GDPR world: I want to avoid the risk of breaches on your side affecting mine. Secondly, today you want to send nice compensation emails, tomorrow (under pressure from your board) you start mining the emails for competitive research and increased monetisation. I wouldn’t want to put the reputation of my service on the line for this.
I couldn't use it for my app as all its functionality needs to work in an airgapped environment. But in any case, I would consider offering a paid tier where you don't use it to send marketing messages.
I also believe that using it in the way you propose could cause you to run foul of GDPR because there isn't a way to separate the marketing message from the app functionality but I don't have a well formed version of that idea yet.