Thunderbird is an excellent e-mail client and has built in e-mail archiving, however one thing that it doesn’t do intuitively is an offline archive. Here’s the situation: you have an IMAP account in Thunderbird and you want to archive some old e-mail offline (take it off of the IMAP server completely). Simply using Thunderbird’s archive feature will create an Archives folder in your IMAP inbox and move everything to there which isn’t exactly what you want. Instead what you need to do is actually move these e-mails to a new location under your Local Folders. Once the move is complete you can verify that they are indeed now stored locally and (optionally) delete them the IMAP account.
Hopefully this helps out anyone else looking for a solution to an offline IMAP archive!