private string FormatMultipleEmailAddresses(string emailAddresses)
{
var delimiters = new[] { ',', ';' };
var addresses = emailAddresses.Split(delimiters, StringSplitOptions.RemoveEmptyEntries);
return string.Join(",", addresses);
}
Now you can use it like
var mailMessage = new MailMessage();
mailMessage.To.Add(FormatMultipleEmailAddresses("[email protected];[email protected],[email protected]"));