I do not recommend that you do that. If you get an error from the server, it is better that you close the connection and restart a new one.
Anyway, for what you are doing I recommend that you use the MIME message class in conjunction with the SMTP class. I has better support for bulk mailing.
If you can use Microsoft Exchange, using the MIME message class with the pickup folder sub-class is an even better solution for bulk mail under Windows and you will have problems like that.
phpclasses.org/mimemessage