As mentioned Milestone provides a free open SDK and API’s across the entire product line which is Essential+ (free up to 8 cameras), Express+ (up to 48 cameras / single site), Pro+ (unlimited cameras, recorders, and sites), and then Expert & Corporate which are for large installs. (Ignore the regular Express & Professional non-plus products as they are the older code-base)
Check out the feature matrix here: https://content.milestonesys.com/l/6a0734c310dcd321/
Because you have access to the rule engine, any condition, status, or input can trigger an output (email being one of them). Milestone will be releasing a new Mobile SDK as well that allows the integration of video (live or stored) into apps, webpages, or other programs across many platforms (Windows, Linux, Web, Android, iOS, etc).
I just installed my first OpenHAB2 instance so I am looking forward to learning about it, as well as writing some plugins or integrations with my Milestone system.