Sometimes you want to install some app from the internet, and your Mac gives you an error that says something like "can't be opened because it's from an unidentified developer" or "...is damaged and can't be opened. You should move it to the Trash."
Well if you trust the app, and want to install it anyways. Just disable macOS Gatekeeper.
Re-enable gatekeeper with: sudo spctl --master-enable
Apple Mail has been a finicky bit of software since its inception. Some love it, some hate it. One problem we've seen recently in V10 with High Sierra is that emails display [the usually] hidden HTML code. This can be frustrating. Some have 'solved' it using Apple Mail's built-in "rebuild" option, but the problem seems to return pretty quick. Here's how you can solve this issue and other weird mail issues too.
We're some guys and gals in San Francisco who fix macs. We can remote in and help you with stuff. It's like our job or whatever.
(five one zero) 761-6559