Until a couple of years ago all my email access was via POP3 and I had quite a comprehensive set of rules that put messages into relevant folders. Of course the mail file was backed up as part of my normal backup procedure.
Now I'm using a smartphone more often IMAP seems to have taken over and I'm not quite sure what is the best way to retain emails. I'm using Thunderbird but I guess most clients would have a Local Folders - is this the best way to retain when I know I'm not likely to want remote access to older emails. It seems cumbersome to have 1000's of emails in my inbox(es).