I'm an IT Manager, computer programmer, web developer, etc. But that's a bit irrelevant since there's little to do wrong when the only thing involved is clicking a download link that purports to "Download all Private Messages as:". If you could give a scenario of what I might be doing wrong that would result in an uncorrupted CSV file that has only Sent Items instead of "all Private Messages", it would be a bit more believable.

Administering a PhpBB forum myself (and a number of other sites with various types of server side scripting software), I would guess there's a problem in the code (probably in the SQL query that retrieves the table to parse) somewhere that is making so that it doesn't retrieve what is intended.

(Could a non-technical person try to download all their PMs and see what happens and post the result?)