Comment by neilv

That imprecise chunking (with MD5 sums, and distributed across an FS tree, and using hash IDs, then separately storing the assembly information) seems like it would be a headache to restore, or to use with other mail programs.

An alternative idea is to properly parse it and store in smaller mbox files, such as one file month, with the idea that any month in the past usually will not change. (And if it changes because they are storing frequently changing attributes in a faux header, like an atime, then maybe strip that header.) Then your incremental `restic` backups work fine, and you can also use it easily with a variety of mail programs (MUAs, impromptu IMAP servers for migration, quick text editor, etc.).