PortBlog.API/Shared/KBR.Shared/Mail/MailHelpers.cs

namespace KBR.Shared.Mail
{
    public static class MailHelpers
    {
        // Replace email with other domain passed as a parameter
        public static string ReplaceEmailDomain(string email, string newDomain)
        {
            if (string.IsNullOrWhiteSpace(email))
                throw new ArgumentException("Email cannot be null or empty.", nameof(email));

            if (string.IsNullOrWhiteSpace(newDomain))
                throw new ArgumentException("New domain cannot be null or empty.", nameof(newDomain));

            var atIndex = email.IndexOf('@');
            if (atIndex == -1)
                throw new ArgumentException("Invalid email format.", nameof(email));

            return $"{email.Substring(0, atIndex + 1)}{newDomain}";
        }
    }
}