I agree with BrowserUk, you are being asked to deal with this totally backwards!

If the volume of submissions is that high then they should contemplate involving IT and have the form submit into a database, then the data can be dealt with there through another interface, or it could be periodically downloaded to the client application.

Typically I have handled this by exporting the data using another small script either into an SQL dump, or a CSV file, zip it up with a password and send it (either by direct download or by email). But have the client contemplate that the better solution - and a less time intensive one for their staff - may well be to approach the problem top down, rather than trying to implement a patchwork solution.

jdtoronto