Hi,
one of the users send fast all his limit of emails (500), but in admin queue I can't find any email sample.
How can I preview one of the sent emails to check if it is spam?
There is no way to see emails content if you have no its copy. So you will need to configure exim to store/send copies of sent emails to a desired location.
I believe the forums contain instructions on how to configure exim for the purposes.