Compatibility fix for Filament v5.7.0+.
Filament v5.7.0 widened InteractsWithActions::unmountAction() from bool $canCancelParentActions = true to bool|string|null $cancelParentActions = null. The narrower override in ListMails no longer matched, so Laravel died during package discovery with:
Declaration of Backstage\Mails\Resources\MailResource\Pages\ListMails::unmountAction(bool $canCancelParentActions = true): void
must be compatible with Filament\Pages\BasePage::unmountAction(string|bool|null $cancelParentActions = null): void
The override now accepts the widest type and forwards only the arguments it actually received, so it works across the whole filament/filament: ^5.0 range — the parent keeps its own default on either signature, and the new "cancel up to a named parent action" string form passes through untouched.
What to do: composer update backstage/mails — no code changes needed on your side.