tl;dr. web hosting && a panic attack && security threat

i wasn't sure whether my brother's domain was hosted or not (because it wasnt showing a website and he didnt know any better).
so i decided to host a react-app for it on netlify and pointed the domain's nameservers towards it (a separate security threat at bottom).
all went well and now when you punch in the domain it ..all-behold.. shows a website.

NOW, i remember my brother was using the domain's email which probably means it was hosted, right?. so im panicking because im not sure whether i just deleted all his emails or not because it's 1:15 am and he's asleep.
there is a rant in there somewhere but im in too much of a shock as to how much data i might have just accidentally deleted
.
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another tl;dr: my domain registrar let me change someone else's settings..
the reason i didnt know his domain settings is that he didnt know his password.
i had bought a couple of domains and was gonna host them on netlify. while i was doing this a bright idea hit me.. "you should finally build a website for your brother for the domain he bought 7 years ago"..
this is where the fun begins.
i sent an email to my registrar to point all nameservers of all domains to my nameservers and just to try out i included my brother's domain into it (i dont own this domain it's not registered by my email), and the next day i get an email telling me they've successfully made all changes.
.
Now tomorrow is monday and i'm going to their office to tell them i found a security flaw and see how long i can stall before actually telling them what it was and how their live's could've been made hell.

recent graduate and fresh into the market with little experience in what i've chosen as a career. got my first (tiny) paycheck for my first project.
didnt know what to do with the money so i bought 3 domains of which 2 are my name with different spellings (i am not a narcissist)

First rant after coming on this website.
Took me 5 seconds to find the search component, GUYS? VISUAL DESCRIPTIONS?