My MacBook Pro is getting old (17" mid-2009, El Capitan 10.11.6), My outgoing email stopped working. After a number attempts to fix, including restoration from TimeMachine backup, all my existing mailboxes are gone. (Mailboxes created by me to sort by subject.) My email on iPhone and iPad are fine. My email on iCloud is fine and complete with all mailboxes.
I'm thinking about doing another TimeMachine restoration from an earlier backup, but I thought I would reach out to the community before taking that step. Any ideas on how to restore all my mailboxes?