If you are using a Microsoft Exchange email server or any system/server that supports Exchange ActiveSync protocol (for example, Outlook.com), there is a "remote search" feature where you can search for messages older than the current "sync horizon". (Currently maximum 30 days) I don't believe that feature exists for IMAP accounts at the present time.
Blackberry has acknowledged this issue (according to what I saw, via a twitter dialogue) and I believe they are looking into offering a longer option as a future update.
As another alternative, there is an official port of the open-source Android K-9 email client in Blackberry World. The Android copy I have here, on an IMAP account, doesn't use a "date" history mechanism, you can set a "number of messages to display" anywhere from 10 to 1000. There is a button labeled "Load up to X more", where "X" is your display quantity. If you configure the account to display 1000 messages, that button is labeled "Load up to 1000 more". I don't know if you can just keep hitting that button indefinitely, I don't have an IMAP account with enough messages in it right now to test that. At some point I'm sure you will run into memory/storage limitations.
I assume that the version in BB World is similiar since it was approved after the latest version of K-9 Mail was released, which is the version I have here on my tablet that I just verified the IMAP settings on. Bear in mind that this email client will not integrate into the Blackberry Hub etc, but it may be useful if the main priority is to see older messages on an IMAP account.
