Put everything into a single notmuchmail database. It wil deduplicate the emails and even provide a fast local search interface for all of your mails https://no

Put everything into a single notmuchmail database. It wil deduplicate the emails and even provide a fast local search interface for all of your mails https://notmuchmail.org/ https://notmuchmail.org/ For gmail you can use lieer to pull everything https://github.com/gauteh/lieer https://github.com/gauteh/lieer (tho it can take hours to pull everything). Fastmail has a similar tool https://github.com/elizagamedev/mujmap https://github.com/elizagamedev/mujmap If you unpack tour mbox into maildir then you can import that too into notmuch After everything is in notmuch you can easily push your mail to fastmail with isync/mbsync https://isync.sourceforge.io/ https://isync.sourceforge.io/ As a bonus point you now also have a easily updatable local backup of your e-mails.