server/src/Core/Utilities/EmailValidation.cs

using System.Net.Mail;
using System.Text.RegularExpressions;
using Bit.Core.Exceptions;
using MimeKit;

namespace Bit.Core.Utilities;

public static class EmailValidation
{
    public static bool IsValidEmail(this string emailAddress)
    {
        if (string.IsNullOrWhiteSpace(emailAddress))
        {
            return false;
        }

        try
        {
            var parsedEmailAddress = MailboxAddress.Parse(emailAddress).Address;
            if (parsedEmailAddress != emailAddress)
            {
                return false;
            }
        }
        catch (ParseException)
        {
            return false;
        }

        // The regex below is intended to catch edge cases that are not handled by the general parsing check above.
        // This enforces the following rules:
        // * Requires ASCII only in the local-part (code points 0-127)
        // * Requires an @ symbol
        // * Allows any char in second-level domain name, including unicode and symbols
        // * Requires at least one period (.) separating SLD from TLD
        // * Must end in a letter (including unicode)
        // See the unit tests for examples of what is allowed.
        var emailFormat = @"^[\x00-\x7F]+@.+\.\p{L}+$";
        if (!Regex.IsMatch(emailAddress, emailFormat))
        {
            return false;
        }

        return true;
    }

    /// <summary>
    /// Extracts the domain portion from an email address and normalizes it to lowercase.
    /// </summary>
    /// <param name="email">The email address to extract the domain from.</param>
    /// <returns>The domain portion of the email address in lowercase (e.g., "example.com").</returns>
    /// <exception cref="BadRequestException">Thrown when the email address format is invalid.</exception>
    public static string GetDomain(string email)
    {
        try
        {
            return new MailAddress(email).Host.ToLower();
        }
        catch (Exception ex) when (ex is FormatException || ex is ArgumentException)
        {
            throw new BadRequestException("Invalid email address format.");
        }
    }
}