Here are some tips on how to reduce the amount of memory that your email files consume:

• The first-time indexing process can consume a lot of RAM.  Index your messages overnight by selecting "Indexing > Index all Folders" from the Tools menu. Then restart Postbox to reset your memory.
• You prevent message bodies from being indexed on an account-by-account basis. Go to Preferences|Options > Accounts > [your account] > Local Storage and deselect indexing for message bodies.
• If you are on IMAP, unsubscribe from folders that are infrequently used by selecting "Subscribe..." from File menu.
• Delete older messages that are no longer needed.
• Empty your Trash and Junk folders.
• Periodically compact some of your larger folders. Additionally, you can set thresholds for when folders should be automatically compressed by going to Preferences|Options > Advanced > Network & Disk Space and set a lower number for compacting.

Additionally, if you are using Gmail:

• Limit the number of messages that appear in IMAP folders. This can be adjusted at Gmail on the Web > Settings > Forwarding and IMAP tab > Folder Size Limits.
• Consider unsubscribing from the All Mail folder, and use a different folder for Archiving.
• Unsubscribe from the Gmail "Important" folder if you are not actively using that label.
• Use Postbox Topics instead of Postbox Labels, as each time you use a Gmail Label it creates another copy of the message in Postbox.