checkAllowEmail(control: FormControl) {
    this.userService.getUserByEmail(control.value)
     .subscribe((user: UserDto) => {
       console.log(user);
       if (user !== undefined) {
         console.log(this.isAllowEmail);
         this.isAllowEmail = false;
         console.log(this.isAllowEmail);
       }
     });
  }

getUserByEmail(email: string): Observable<UserDto> {
    return this.http.get(`http://localhost:9092/api/v1/users?email=${email}`)
      .map((response: Response) => response.json())
      .map((user: UserDto) => user ? user : undefined);
  }

export class UserDto {
  constructor(
    public email: string,
    public name: string,
    public role: string
  ) {}
}

{"name":"art","email":"art@mail.ru","role":"user"}