> The other option is to have a column on the mailbox table to flag that
> it is a default_mailbox -- but then I'd have to ensure there's only
> one column for each "person" flagged that way.

- is_default BOOL column in mailbox table
- conditional index :

UNIQUE INDEX ON mailboxes( owner ) WHERE is_default = 't'

> Second question. So, after a while the obvious problem happens and
> users have too many mailboxes and they want a way to group them into

Looks like a tree. Why not use a LTREE ?

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