You should rather use Smtp.SendMessage method overload, that takes SmtpMail instance as parameter:
public SendMessageResult SendMessage(SmtpMail smtpMail)
You can modify SmtpMail instance according to your needs before each send. You can add/remove items from SmtpMail.To collection and use Smtp for actual sending.
IMail email = ...
SmtpMail smtpMail = SmtpMail.CreateFrom(email);
string first = smtpMail.To[0];
smtpMail.To.Clear();
smtpMail.To.Add(first);
using(Smtp smtp = new Smtp())
{
smtp.Connect("smtp.server.com"); // or ConnectSSL for SSL
smtp.UseBestLogin("user", "password");
smtp.SendMessage(smtpMail);
smtp.Close();
}