* to be able to switch MUAs without losing their mail data, addressbook data,
or other account settings. (This covers the case where the user has more than
one MUA installed on a given machine at a given time.) On the other hand,
they may want to have distinct accounts on distinct MUAs, so this does not
imply that all MUAs must have the same settings. Tricky.
* to be able to export mail messages from one MUA and import them into
another, potentially maintaining whatever "folder" structuring was present,
as they do with .zip archives (or similar) and ordinary files. (This covers
the case where the user wants to migrate an existing mail repository from one
computer to another, or export a number of messages from the repository for
I've done enough mailbox migrations that these wants are personal. They may or
may not imply the need for a common "mailbox" format (probably better
described as a "mail archive" format, so that implementers don't feel they
need to store their "live" data in any particular way).