A bit difficult to answer since I have no idea how your system operates.
However, there are most likely numerous ways. A simple example would be to write a line do DB, file or whatever each time a mail is sent. I.e.;
<?
// send mail code
// insert some dummy entry into a db table, or increase a counter somewhere.
?>
a separate script, loaded in i.e. a javascript popup would read this data and determine the progress. I.e., before launch, count the number of total e-mails to be sent, pass this on to the second script, compare values, generate statistics, bars or whatever you want. This script would have to be refreshed every couple of seconds (using a meta-tag for example). (When refreshing, pass on the total number of mails, and re-read database, file or whatever to build updated statistics).
Bad explanation, but hopefully you get the point ;p
