A lot of folks have sought solutions to the “Previous Recipients” problem in Apple Mail. The problem arises because the data is stored in the AddressBook's app support folder, not Mail's, and even then it's in SQL database format, which is not directly readable as text (*+_SHAME on Apple_+* for creating that mess!). Here is the definitive solution for Mac OS X.6; it ain’t pretty, but it’s not difficult either!
1. Download SQLite Database Browser fromhttp://surfnet.dl.sourceforge.net/project/sqlitebrowser/sqlitebrowser/2.0%20beta 1/sqlitebrowser200_b1osx.zip Okay, it's probably written in Java--ugly, un-Maclike, unintuitive...but it does work.
2. Double-click it to unZIP it, move the unZIPed application into a suitable folder (e.g. Applications or Utilities), and trash the ZIP file.
3. Navigate to your user’s Library/Application Support/AddressBook folder (that’s right, it’s in the AddressBook folder, not an Apple Mail folder--shame, Apple!), duplicate the file ‘MailRecents-v4.abcdmr’, and move it to a convenient location.
4. Drop the duplicate file onto SQLite Database Browser, select the ‘Browse Data’ tab, then choose table ‘ZABCDMAILRECENT’. This is your Apple Mail ‘Previous Recipients’ data.
5. Choose ‘File/Export/Table as CSV file…’, choose ‘ZABCDMAILRECENT’ from the pop-up, click ‘Export’, choose a convenient location and file name, and click Save.
6. Your data has now been saved as a ‘comma-separated variable’ file which is readily opened, either as plain text or as data, in an office suite or spreadsheet program such as OpenOffice, Numbers, Google Docs, or Excel, where you can delete extraneous data and sort what remains as desired. You can then save it again in the CSV format, or in some other format, possibly substituting tabs for the commas as the separator, thus giving you a tabbed file (e.g., first name, tab, last name, tab, email address), or as plain text.
Make sure the file you save ends with the CSV extension (.csv)