When building an SMS app, there are some "reserved" keywords, such as HELP and STOP that are commonly expected to perform a certain function.
MMA (Mobile Marketing Association) has published the following best practices: http://mmaglobal.com/bestpractices.pdf
After reading the document, I made a list of the following reserved keywords that should be handled:
I have also seen services treating curse words as an opt-out request.
What other keywords should be reserved and handled globally? What is the expected behavior?
We see a number of developers also implementing Cancel, Unsubscribe, Unsub, and Quit as synonyms for opting out. We also encourage developers to respond to the vowel-less versions of keywords. For example, STP for "stop" and "HLP" for "help". We also suggest that you implement "{keyword} ALL" since some shared shortcodes use that syntax and your users may be expecting it to work.
It's just good UI design - anticipate what your users are going to attempt, and then respond to that. It's also a good idea to have some analytics on what people send to your SMS application -- especially if that input is rejected. You can analyze this for commonly-used commands that you're unable to respond to.