> my email is not a string, it's a email

Here's the documentation for custom types in F#:

http://fsharpforfunandprofit.com/posts/conciseness-type-defi...

http://fsharpforfunandprofit.com/series/understanding-fsharp...

http://fsharpforfunandprofit.com/series/designing-with-types...

Types are a very good way to model your data strictly.