We’ve detected an issue where iOS’ Sparrow mail client is unable to see any mail folders other than the Inbox. We are working on tracking the particulars down and getting this resolved, with bug reports opened with both our mail server vendor as well as Sparrow.
In the mean time, iOS’ built-in mail client is fully supported using either IMAP or ActiveSync (Exchange).
Please accept our apologies for the inconvenience.
UPDATE: This issue has now been resolved.