Mass email - how does it work?

Mass email - how does it work?

I am using mass email to send a newsletter to about 4000+ customers.

Since the daily limit is 500 I will be sending them out over several days. I used a manual criteria to find the customers and noticed that there is no obvious way that the contacts are sorted. They are not listed alphabetically or by any field I can see on the page; it seems to change randomly every time I search the criteria. This means I have no way of telling which contacts have already been sent the newsletter.

Is there some way to change the way the manual criteria view is listed?

Also, how can I ensure that the same contacts are not sent the same newsletter twice?