It is possible to move your TouchDown data profile to the SD card, but note this is a bit risky. If you tether the phone to a computer and start browsing files, you could corrupt and reset the TouchDown database.
To move the data to SD card,
NOTE: if your server security policy requires encryption you will not see this option.
We also caution you to store as little data on the device as possible. Unusually large databases have their own host of errors, and synchronizing less data can often improve performance. If your server supports it, server search is an excellent way to browse Exchange emails that are not synced to the device. This should allow users the freedom to synchronize fewer folders or fewer days of email if space is an issue.