Say which kind of mail leaves from which mailbox

app/Services/Mail/MailPurpose.php

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+<?php
+
+namespace App\Services\Mail;
+
+/**
+ * What CluPilot sends, as a fixed list.
+ *
+ * Code and not data, for the same reason SecretVault::REGISTRY is curated: a
+ * mapping that can name anything cannot be checked, and a purpose without a
+ * sender would only be noticed at runtime — as a mail that did not arrive.
+ *
+ * SYSTEM is the fallback. Not a separate "which is the default" switch, which
+ * could contradict the mapping; it is a row of the same table and is edited
+ * like any other.
+ */
+final class MailPurpose
+{
+    public const MAINTENANCE = 'maintenance';
+    public const PROVISIONING = 'provisioning';
+    public const SUPPORT = 'support';
+    public const BILLING = 'billing';
+    public const SYSTEM = 'system';
+
+    /** @var array<int, string> */
+    public const ALL = [
+        self::MAINTENANCE,
+        self::PROVISIONING,
+        self::SUPPORT,
+        self::BILLING,
+        self::SYSTEM,
+    ];
+
+    /** The settings key holding the mailbox key for a purpose. */
+    public static function settingKey(string $purpose): string
+    {
+        return 'mail.purpose.'.$purpose;
+    }
+}

app/Services/Mail/MailboxResolver.php

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+<?php
+
+namespace App\Services\Mail;
+
+use App\Models\Mailbox;
+use App\Support\Settings;
+use InvalidArgumentException;
+
+/**
+ * Which mailbox a given kind of mail goes out from.
+ *
+ * Read on every resolve rather than cached in a property: a queue worker lives
+ * for hours, and a mapping cached at construction would keep sending from the
+ * address that was configured when the worker started.
+ */
+final class MailboxResolver
+{
+    public function for(string $purpose): ?Mailbox
+    {
+        if (! in_array($purpose, MailPurpose::ALL, true)) {
+            throw new InvalidArgumentException("Unknown mail purpose [{$purpose}].");
+        }
+
+        $box = $this->named(Settings::get(MailPurpose::settingKey($purpose)));
+
+        if ($box !== null) {
+            return $box;
+        }
+
+        // Fall back rather than fail: a mail that goes out from the wrong
+        // address is recoverable, one that is never sent may not be noticed.
+        return $purpose === MailPurpose::SYSTEM
+            ? null
+            : $this->named(Settings::get(MailPurpose::settingKey(MailPurpose::SYSTEM)));
+    }
+
+    private function named(mixed $key): ?Mailbox
+    {
+        return is_string($key) && $key !== '' ? Mailbox::findByKey($key) : null;
+    }
+}

tests/Feature/Mail/MailboxResolverTest.php

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+<?php
+
+use App\Models\Mailbox;
+use App\Services\Mail\MailboxResolver;
+use App\Services\Mail\MailPurpose;
+use App\Support\Settings;
+
+beforeEach(function () {
+    config()->set('admin_access.secrets_key', 'base64:'.base64_encode(random_bytes(32)));
+});
+
+it('resolves a purpose to the mailbox it points at', function () {
+    Mailbox::factory()->create(['key' => 'support', 'address' => 'support@clupilot.com']);
+    Settings::set('mail.purpose.support', 'support');
+
+    expect(app(MailboxResolver::class)->for(MailPurpose::SUPPORT)?->address)
+        ->toBe('support@clupilot.com');
+});
+
+it('falls back to system when a purpose has no mailbox', function () {
+    Mailbox::factory()->create(['key' => 'no-reply', 'address' => 'no-reply@clupilot.com']);
+    Settings::set('mail.purpose.system', 'no-reply');
+
+    // billing deliberately unset
+    expect(app(MailboxResolver::class)->for(MailPurpose::BILLING)?->address)
+        ->toBe('no-reply@clupilot.com');
+});
+
+it('falls back to system when the named mailbox no longer exists', function () {
+    Mailbox::factory()->create(['key' => 'no-reply', 'address' => 'no-reply@clupilot.com']);
+    Settings::set('mail.purpose.system', 'no-reply');
+    Settings::set('mail.purpose.billing', 'geloescht');
+
+    expect(app(MailboxResolver::class)->for(MailPurpose::BILLING)?->address)
+        ->toBe('no-reply@clupilot.com');
+});
+
+it('returns null rather than guessing when even system is unset', function () {
+    expect(app(MailboxResolver::class)->for(MailPurpose::SYSTEM))->toBeNull();
+});
+
+it('rejects a purpose that is not in the curated list', function () {
+    expect(fn () => app(MailboxResolver::class)->for('erfunden'))
+        ->toThrow(InvalidArgumentException::class, 'erfunden');
+});