Q Could some problems of ‘lost’ addresses in Mail result from failure to migrate the ‘previous addresses’ which are stored by the Mail app?
A Theoretically this could be the case.
Mail has access to two different lists of email addresses: those stored normally in Contacts (formerly Address Book), and those accessed using the Previous Recipients command in the Window menu. To be sure that entries in the latter list are stored securely, it might be wise to periodically open that list, select all important entries, and click on the Add to Address Book button.
Address books are now stored in sandboxed ‘containers’. To locate yours, browse the Library folder in your Home folder (~/Library) by holding the Opt key and opening the Finder’s Go menu: you will then see Library listed as one of the locations.
In ~/Library, address books are stored in the path Containers/com.apple.AddressBook/Data/Library/Application Support/AddressBook. Currently this is an alias which points at the simpler path of ~/Library/Application Support/AddressBook, but that could change at any time. There you should find straight AddressBook files, MailRecents files, and possibly Migration files too.
If you have apparently lost addresses from Mail as the result of a migration, you could try replacing the MailRecents file(s), perhaps.
Updated from the original, which was first published in MacUser volume 26 issue 25, 2010.