diff --git a/app/Support/Settings.php b/app/Support/Settings.php
index 76bb8a7..876dec8 100644
--- a/app/Support/Settings.php
+++ b/app/Support/Settings.php
@@ -64,4 +64,22 @@ final class Settings
     {
         return (bool) self::get($key, $default);
     }
+
+    /**
+     * Remove a setting entirely, falling back to the caller's own default
+     * from then on.
+     *
+     * Not `set($key, null)`: that would still leave a row (and a cache
+     * entry) behind — indistinguishable to `get()`, but a migration's
+     * `down()` undoing its own `up()` should leave no row, not a null one.
+     * Deletes the row before dropping the cache, the same order `set()`
+     * writes-then-invalidates in — a reader between the two statements sees
+     * the OLD value from a cache that is about to be dropped, never a stale
+     * one that outlives the row it described.
+     */
+    public static function forget(string $key): void
+    {
+        DB::table('app_settings')->where('key', $key)->delete();
+        Cache::forget(self::CACHE_PREFIX.$key);
+    }
 }
diff --git a/database/migrations/2026_07_28_100000_seed_mailboxes_from_environment.php b/database/migrations/2026_07_28_100000_seed_mailboxes_from_environment.php
index 7ed0db4..4d6d3f0 100644
--- a/database/migrations/2026_07_28_100000_seed_mailboxes_from_environment.php
+++ b/database/migrations/2026_07_28_100000_seed_mailboxes_from_environment.php
@@ -13,6 +13,13 @@ use Illuminate\Support\Facades\DB;
  *
  * Empty rather than absent: a page that lists what is expected tells the owner
  * what is still missing. An empty list would only say "nothing here".
+ *
+ * Safe to run more than once: `Mailbox::firstOrNew()` rather than `new
+ * Mailbox()`, because a migration that fails partway leaves the migrations
+ * table not recording it, and a retry must update the rows already written
+ * rather than collide with them on the unique key. `up()` and `down()` are
+ * each wrapped in their own transaction for the same reason — MariaDB does
+ * not get one automatically the way Postgres does.
  */
 return new class extends Migration
 {
@@ -25,11 +32,49 @@ return new class extends Migration
         'info' => ['no_reply' => false, 'purposes' => []],
     ];
 
+    /**
+     * config/mail.php's own fallbacks when MAIL_HOST/MAIL_PORT are unset.
+     *
+     * config('mail.mailers.smtp.host') cannot tell "genuinely unset" apart
+     * from "deliberately set to 127.0.0.1" — config/mail.php's own
+     * env('MAIL_HOST', '127.0.0.1') has already substituted by the time this
+     * migration reads it, and 127.0.0.1 is not '', so it would sail straight
+     * past MailboxTransport's "host is not configured" guard (Task 4) and
+     * quietly dial localhost instead of refusing loudly.
+     *
+     * Read raw env() instead? That returns null (correctly distinguishing
+     * "unset") ONLY as long as .env was actually loaded — which Laravel skips
+     * entirely once `config:cache` has run (LoadEnvironmentVariables checks
+     * configurationIsCached() first). deploy/update.sh always migrates
+     * before it rebuilds the cache, but nothing enforces that order, and
+     * env() is also awkward to test (it does not go through config(), so
+     * Pest's usual config()->set() cannot drive it). Comparing against the
+     * placeholder keeps this correct under config:cache regardless of
+     * ordering, and testable the same way every other test in this suite
+     * already is. Trade-off: an install that deliberately runs its relay on
+     * 127.0.0.1:2525 reads as unset — nobody does that for outbound mail.
+     */
+    private const UNCONFIGURED_HOST = '127.0.0.1';
+
+    private const UNCONFIGURED_PORT = 2525;
+
     public function up(): void
     {
-        // The server, once. Whatever the .env already proved workable.
-        Settings::set('mail.host', (string) config('mail.mailers.smtp.host', ''));
-        $port = (int) config('mail.mailers.smtp.port', 587);
+        DB::transaction(function () {
+            $this->seed();
+        });
+    }
+
+    private function seed(): void
+    {
+        // The server, once. Whatever .env already proved workable — but not
+        // config/mail.php's own placeholder (see UNCONFIGURED_HOST above).
+        $configuredHost = (string) config('mail.mailers.smtp.host', '');
+        $host = $configuredHost === self::UNCONFIGURED_HOST ? '' : $configuredHost;
+        Settings::set('mail.host', $host);
+
+        $configuredPort = (int) config('mail.mailers.smtp.port', 0);
+        $port = $configuredPort === self::UNCONFIGURED_PORT ? 0 : $configuredPort;
         Settings::set('mail.port', $port);
 
         // NOT copied from MAIL_SCHEME. That variable currently reads "tls",
@@ -47,8 +92,9 @@ return new class extends Migration
             ? substr($configuredUser, strpos($configuredUser, '@') + 1)
             : 'clupilot.com';
 
-        // A password may already be stored in the vault; it moves rather than
-        // being left behind in a registry entry that is about to disappear.
+        // A password may already be stored in the vault. It is CARRIED, not
+        // moved: the app_secrets row is deliberately left in place below
+        // rather than deleted, so this block only ever reads it.
         $storedPassword = DB::table('app_secrets')->where('key', 'mail.password')->value('value');
         $envPassword = (string) config('mail.mailers.smtp.password', '');
 
@@ -63,8 +109,8 @@ return new class extends Migration
         foreach (self::SEED as $key => $meta) {
             $isConfigured = $key === 'no-reply' && $configuredUser !== '';
 
-            $box = new Mailbox([
-                'key' => $key,
+            $box = Mailbox::firstOrNew(['key' => $key]);
+            $box->fill([
                 'address' => $isConfigured ? $configuredUser : $key.'@'.$domain,
                 'display_name' => 'CluPilot',
                 'no_reply' => $meta['no_reply'],
@@ -72,8 +118,27 @@ return new class extends Migration
             ]);
 
             // Only the account that actually exists gets credentials.
-            if ($isConfigured && $envPassword !== '' && $canEncrypt) {
-                $box->password = $envPassword;
+            if ($isConfigured && $envPassword !== '') {
+                if ($canEncrypt) {
+                    $box->password = $envPassword;
+                } else {
+                    // Skipping beats crashing the migration (that is what
+                    // $canEncrypt guards above) — but a skip that LOOKS like
+                    // success is its own failure mode: migrate prints DONE,
+                    // MAIL_MAILER is still log today, and the first real send
+                    // after it flips to smtp throws for every purpose,
+                    // discovered by a customer rather than by this line.
+                    // There is no console page for mailboxes yet, so stdout
+                    // during migrate is the only place an operator can see
+                    // this. echo, not fwrite(STDOUT, ...): both reach the
+                    // real terminal on a plain `php artisan migrate`, but
+                    // only echo goes through PHP's output buffer, which is
+                    // what makes it possible to assert on in a test.
+                    echo '  ! SECRETS_KEY is not set - the SMTP password configured in .env for '
+                        .$configuredUser.' was NOT copied into the mailbox table. Generate SECRETS_KEY, '
+                        .'then either set mailboxes.password directly or migrate:rollback and re-run '
+                        ."this migration.\n";
+                }
             }
 
             $box->save();
@@ -89,17 +154,45 @@ return new class extends Migration
             }
         }
 
-        DB::table('app_secrets')->where('key', 'mail.password')->delete();
+        // mail.password's app_secrets row is deliberately left alone here —
+        // not deleted. Its SecretVault::REGISTRY entry is gone (this task's
+        // other half), so nothing can read or write it through the vault
+        // anymore, but destroying the only remaining copy of a real
+        // credential in the SAME migration that first introduces its new,
+        // less-proven home is backwards for what a stored SMTP password is.
+        // Two ways that unconditional delete used to lose it for real,
+        // reproduced against MariaDB (see the task report):
+        //   - MAIL_USERNAME empty makes $isConfigured false for every key
+        //     above, so the carry-over above never runs — deleting here
+        //     regardless would destroy a rotated password with no copy left
+        //     anywhere.
+        //   - migrate, then migrate:rollback, then migrate again: down()
+        //     deletes the mailbox rows, which would be the ONLY remaining
+        //     copy once this row was already gone.
+        // Left in place, a rollback can never lose the credential — at worst
+        // it is an inert leftover row nothing can reach through
+        // SecretVault::assertKnown() any more. A later migration can remove
+        // it once the mailbox takeover has been proven on a live install.
     }
 
     public function down(): void
     {
-        DB::table('mailboxes')->whereIn('key', array_keys(self::SEED))->delete();
+        DB::transaction(function () {
+            DB::table('mailboxes')->whereIn('key', array_keys(self::SEED))->delete();
 
-        foreach (MailPurpose::ALL as $purpose) {
-            DB::table('app_settings')->where('key', MailPurpose::settingKey($purpose))->delete();
-        }
+            // Settings::forget(), not a raw DB::table(...)->delete(): the
+            // cache is Cache::forever()'d with no TTL, so bypassing it here
+            // would leave Settings::get() returning what this loop just
+            // deleted until something else happens to write the same key.
+            // Reproduced: immediately after a raw-delete rollback,
+            // Settings::get('mail.purpose.system') still answered 'no-reply'.
+            foreach (MailPurpose::ALL as $purpose) {
+                Settings::forget(MailPurpose::settingKey($purpose));
+            }
 
-        DB::table('app_settings')->whereIn('key', ['mail.host', 'mail.port', 'mail.encryption'])->delete();
+            foreach (['mail.host', 'mail.port', 'mail.encryption'] as $key) {
+                Settings::forget($key);
+            }
+        });
     }
 };
diff --git a/tests/Feature/Mail/MailboxSeedMigrationTest.php b/tests/Feature/Mail/MailboxSeedMigrationTest.php
new file mode 100644
index 0000000..4c3e822
--- /dev/null
+++ b/tests/Feature/Mail/MailboxSeedMigrationTest.php
@@ -0,0 +1,216 @@
+<?php
+
+use App\Models\Mailbox;
+use App\Services\Secrets\SecretCipher;
+use App\Support\Settings;
+use Illuminate\Support\Facades\DB;
+
+/**
+ * Loads a fresh instance of the seed migration's anonymous class every call.
+ * `require`, not `require_once`: PHP re-evaluates the `return new class ...`
+ * expression each time, so every call gets its own instance — this is what
+ * lets a test call ->up() with config the REAL migrate run (already applied
+ * once by RefreshDatabase before any test's own beforeEach) could not have
+ * used, to reach branches nothing else exercises.
+ */
+function loadMailboxSeedMigration(): object
+{
+    return require database_path('migrations/2026_07_28_100000_seed_mailboxes_from_environment.php');
+}
+
+beforeEach(function () {
+    config()->set('admin_access.secrets_key', 'base64:'.base64_encode(random_bytes(32)));
+});
+
+it('does not copy MAIL_SCHEME through — a Symfony transport only knows smtp and smtps', function () {
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.scheme', 'not-a-symfony-scheme');
+    config()->set('mail.mailers.smtp.port', 587);
+
+    loadMailboxSeedMigration()->up();
+
+    expect(Settings::get('mail.encryption'))->toBe('tls');
+});
+
+it('derives ssl from port 465, regardless of what scheme is configured', function () {
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.scheme', 'irrelevant');
+    config()->set('mail.mailers.smtp.port', 465);
+
+    loadMailboxSeedMigration()->up();
+
+    expect(Settings::get('mail.encryption'))->toBe('ssl')
+        ->and(Settings::get('mail.port'))->toBe(465);
+});
+
+it('treats config/mail.php\'s own 127.0.0.1 / 2525 defaults as unset, not as a real relay', function () {
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.host', '127.0.0.1');
+    config()->set('mail.mailers.smtp.port', 2525);
+
+    loadMailboxSeedMigration()->up();
+
+    // '' / 0 is exactly what MailboxTransport's guards treat as "not
+    // configured" — 127.0.0.1 is not '', so it would have sailed straight
+    // past the Task 4 guard and quietly dialled localhost instead.
+    expect(Settings::get('mail.host'))->toBe('')
+        ->and(Settings::get('mail.port'))->toBe(0);
+});
+
+it('still stores a deliberately configured host and port untouched', function () {
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.host', 'smtp.example.test');
+    config()->set('mail.mailers.smtp.port', 587);
+
+    loadMailboxSeedMigration()->up();
+
+    expect(Settings::get('mail.host'))->toBe('smtp.example.test')
+        ->and(Settings::get('mail.port'))->toBe(587);
+});
+
+it('carries the vault password across as ciphertext, never decrypting and re-encrypting it', function () {
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.username', 'no-reply@clupilot.com');
+
+    $ciphertext = app(SecretCipher::class)->encrypt('rotated-through-the-console');
+    DB::table('app_secrets')->insert([
+        'key' => 'mail.password', 'value' => $ciphertext, 'created_at' => now(), 'updated_at' => now(),
+    ]);
+
+    loadMailboxSeedMigration()->up();
+
+    $stored = DB::table('mailboxes')->where('key', 'no-reply')->value('password');
+
+    // Byte-for-byte, not just "decrypts to the same plaintext": AES-CBC's
+    // random IV means re-encrypting the SAME plaintext produces a DIFFERENT
+    // ciphertext string, so this is what actually discriminates "carried
+    // as-is" from "decrypted and re-encrypted" — the migration's own claim,
+    // never exercised by any test before this one.
+    expect($stored)->toBe($ciphertext)
+        ->and(app(SecretCipher::class)->decrypt($stored))->toBe('rotated-through-the-console');
+});
+
+it('leaves the vault row in the database even after a normal, fully successful carry-over', function () {
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.username', 'no-reply@clupilot.com');
+
+    $ciphertext = app(SecretCipher::class)->encrypt('rotated-through-the-console');
+    DB::table('app_secrets')->insert([
+        'key' => 'mail.password', 'value' => $ciphertext, 'created_at' => now(), 'updated_at' => now(),
+    ]);
+
+    loadMailboxSeedMigration()->up();
+
+    expect(DB::table('app_secrets')->where('key', 'mail.password')->value('value'))->toBe($ciphertext);
+});
+
+it('does not destroy a stored vault password when MAIL_USERNAME is empty, because it never lands anywhere', function () {
+    // Reproduces the Critical finding: $isConfigured is false for every key
+    // when MAIL_USERNAME is empty, so the carry-over never runs — the old
+    // code deleted the vault row unconditionally regardless.
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.username', '');
+
+    $ciphertext = app(SecretCipher::class)->encrypt('nowhere-to-go');
+    DB::table('app_secrets')->insert([
+        'key' => 'mail.password', 'value' => $ciphertext, 'created_at' => now(), 'updated_at' => now(),
+    ]);
+
+    loadMailboxSeedMigration()->up();
+
+    expect(DB::table('app_secrets')->where('key', 'mail.password')->value('value'))->toBe($ciphertext)
+        ->and(Mailbox::findByKey('no-reply')->getRawOriginal('password'))->toBeNull();
+});
+
+it('survives a migrate, rollback, migrate-again cycle without losing the credential', function () {
+    // Reproduces the other Critical finding: down() deletes the mailbox
+    // rows, which is where the ONLY copy of the password would end up if the
+    // vault row had been deleted (the old behaviour) during the first up().
+    clearMailboxSeed();
+    config()->set('mail.mailers.smtp.username', 'no-reply@clupilot.com');
+
+    $ciphertext = app(SecretCipher::class)->encrypt('rotated-through-the-console');
+    DB::table('app_secrets')->insert([
+        'key' => 'mail.password', 'value' => $ciphertext, 'created_at' => now(), 'updated_at' => now(),
+    ]);
+
+    $migration = loadMailboxSeedMigration();
+    $migration->up();
+    $migration->down();
+    $migration->up();
+
+    expect(DB::table('mailboxes')->where('key', 'no-reply')->value('password'))->toBe($ciphertext);
+});
+
+it('is safe to run twice — a migrations-table drift must not crash on the unique key', function () {
+    // The real baseline from RefreshDatabase's own migrate is already there
+    // (nothing cleared it) — calling up() again must update those rows in
+    // place, not attempt a second INSERT under the same key.
+    //
+    // A plain try/catch, NOT expect(fn () => ...)->not->toThrow(Throwable::class):
+    // verified directly that the latter is not reliable here — a closure that
+    // unconditionally throws still passes ->not->toThrow(Throwable::class) in
+    // this Pest version, which would have made this test pass whether or not
+    // the migration actually crashed.
+    $threw = null;
+    try {
+        loadMailboxSeedMigration()->up();
+    } catch (Throwable $e) {
+        $threw = $e;
+    }
+
+    expect($threw)->toBeNull();
+    expect(Mailbox::query()->pluck('key')->sort()->values()->all())
+        ->toBe(['billing', 'info', 'no-reply', 'office', 'support']);
+});
+
+it('un-seeds on rollback, and does not leave the settings cache holding what it just deleted', function () {
+    // Against the real baseline again — this is what a real rollback tears
+    // down. Warms the cache first the same way a real request would, so a
+    // migration that bypassed Settings::forget() (a raw DB delete) and left
+    // the Cache::forever() entry behind would still read back the old value.
+    expect(Settings::get('mail.purpose.system'))->toBe('no-reply');
+
+    loadMailboxSeedMigration()->down();
+
+    expect(Mailbox::query()->count())->toBe(0)
+        ->and(Settings::get('mail.purpose.system'))->toBeNull()
+        ->and(Settings::get('mail.host'))->toBeNull();
+});
+
+it('prints an operator-visible warning when SECRETS_KEY is missing and a .env password had to be skipped', function () {
+    clearMailboxSeed();
+    config()->set('admin_access.secrets_key', ''); // canEncrypt false
+    config()->set('mail.mailers.smtp.username', 'no-reply@clupilot.com');
+    config()->set('mail.mailers.smtp.password', 'dummy-nicht-echt');
+
+    ob_start();
+    loadMailboxSeedMigration()->up();
+    $output = ob_get_clean();
+
+    expect($output)->toContain('SECRETS_KEY')
+        ->toContain('no-reply@clupilot.com');
+});
+
+it('stays silent when there is no .env password to skip in the first place', function () {
+    clearMailboxSeed();
+    config()->set('admin_access.secrets_key', ''); // canEncrypt false
+    config()->set('mail.mailers.smtp.username', 'no-reply@clupilot.com');
+    config()->set('mail.mailers.smtp.password', '');
+
+    ob_start();
+    loadMailboxSeedMigration()->up();
+    $output = ob_get_clean();
+
+    expect($output)->toBe('');
+});
+
+it('Settings::forget removes the row and does not leave a stale cache entry behind', function () {
+    Settings::set('probe.key', 'value');
+    expect(Settings::get('probe.key'))->toBe('value');
+
+    Settings::forget('probe.key');
+
+    expect(Settings::get('probe.key'))->toBeNull()
+        ->and(DB::table('app_settings')->where('key', 'probe.key')->exists())->toBeFalse();
+});
diff --git a/tests/Feature/Mail/MailboxTakeoverTest.php b/tests/Feature/Mail/MailboxTakeoverTest.php
index 1971677..e5381c7 100644
--- a/tests/Feature/Mail/MailboxTakeoverTest.php
+++ b/tests/Feature/Mail/MailboxTakeoverTest.php
@@ -16,8 +16,14 @@ it('marks no-reply as unanswerable and the rest as answerable', function () {
 });
 
 it('points every purpose at a mailbox, so nothing sends from nowhere', function () {
+    // Not just "the setting has a value" — a purpose pointing at a key with
+    // no matching row would still pass a not-null check on the setting while
+    // MailboxResolver::named() resolves it to nothing and every send throws.
     foreach (MailPurpose::ALL as $purpose) {
-        expect(Settings::get(MailPurpose::settingKey($purpose)))->not->toBeNull();
+        $key = Settings::get(MailPurpose::settingKey($purpose));
+
+        expect($key)->not->toBeNull()
+            ->and(Mailbox::findByKey($key))->not->toBeNull();
     }
 });