Typescript : underscore convention for members

Just name your private variables as you want but don't use _. You could create your own standard and stick to it.

Setters and getters are like any other functions so you can follow the method naming convention.

Do not use "_" as a prefix for private properties.
Use whole words in names when possible.

This is a subjective opinion, feel free to use _ if you must.

Edit: $ can be used to prefix variable names. In my everyday use case I use it in prefixing observable (rxJS) variables.

Edit:

In the case where you have getters then you can use _ to name the field to avoid name conflict.

Using underscores as a prefix or suffix in variable names at all is generally not regarded as good practice.

See the Google Typescript Style Guide. This guide is intended as a prescriptive guide on style, unlike the Microsoft Style Guide linked in another answer which is primarily intended for contributors to the TypeScript repository.

_ prefix/suffix: Identifiers must not use _ as a prefix or suffix

https://google.github.io/styleguide/tsguide.html

Underscore "_" prefix for private fields is out-of-date style. It's better to name your variable in a readable and friendly way.

See Microsoft Typescript coding convention here

6. Do not use "_" as a prefix for private properties.

Those who say, must not use the "_", for them, here is some code from TypeScript site:

class Employee {
    private _fullName: string;

    get fullName(): string {
        return this._fullName;
    } 
    this._fullName = ......
}

Same question on Stackoverflow, you should have a look on it, especially the answer.

For the time being, if accepted, we should not use _, then what are other better ways?

Let's take your example of email, if we will not use _ then, we will come something like this:

member: to,      get/set: emailTo
member: from     get/set: emailFrom 

might be you can think some better name, but every time you need to think, which is not very common in the developer world!

Using _ and the same name for a property is easy to check the code otherwise we will be keep mapping which property to which member.

BUT: If you have been forced by your lead at your company then you can use $ for member and property without it; not a rule but easy way:

class Employee {
    private fullName$: string;

    get fullName(): string {
        return this.fullName$;
    } 
    this.fullName$ = ......
}

The Choice Is Yours!!!