public class RegisterViewModel
{
    [Required]
    [EmailAddress]
    [Remote(action: nameof(AccountController.IsEmailNotInUseAndDomainValid), controller: "Account")]
    [ValidEmailDomain("gmail.com", ErrorMessage = "Email domain must be 'gmail.com'")]
    public string Email { get; set; }

    // ...
}

public class AccountController : Controller
{
    // ...

    [AcceptVerbs("Get", "Post")]
    [AllowAnonymous]
    public async Task<IActionResult> IsEmailNotInUseAndDomainValid(string email)
    {
        var isEmailInUse = await IsEmailNotInUseAsync(email);
        if (isEmailInUse.Value is bool notInUse && notInUse != true || !(isEmailInUse.Value is bool))
            return isEmailInUse;

        var isDomainValid = IsDomainValid(email);
        if (isDomainValid.Value is bool valid && valid != true || !(isDomainValid.Value is bool))
            return isDomainValid;

        return Json(true);
    }

    public async Task<JsonResult> IsEmailNotInUseAsync(string email)
    {
        var user = await _userManager.FindByEmailAsync(email);
        return user == null
            ? Json(true)
            : Json($"Email {email} is already in use.");
    }

    public JsonResult IsDomainValid(string email)
    {
        var validEmailDomainAttribute = typeof(RegisterViewModel).GetTypeInfo()?
            .GetProperty(nameof(RegisterViewModel.Email))?.GetCustomAttribute<ValidEmailDomainAttribute>();
        return validEmailDomainAttribute?.IsValid(email) switch
        {
            true => Json(true),
            false => Json(validEmailDomainAttribute.ErrorMessage),
            _ => throw new NullReferenceException()
        };
    }

    // ...
}