Word 2007 Mail Merge - maximum amount of emails to send to?

Looking for Help with Mail merge through Word 2007. I downloaded a csv from
our database and used mail merge wizard through all the steps. I tested with
5 and then 10 emails and they all sent perfectly.
The trouble came when I downloaded a csv with 20k emails/records. I tested
sending 1-1000 records and it appeared to send fine. Did that for all the
rest of the emails. The Problem comes when looking at my Outlook Sent Items
-- After all was said and done looked at my sent box the next day and out of
all the sends, appears only 300 emails were sent?? Is there a maximum number
of records you can send at a time? Or download in the csv at a time?? Or
even just all could of sent and just not showing up in in Outlook? Any words
of advise or wisdom? thanks in advance.