feat(storage): add comprehensive tests for StorageManager with memory, filesystem, and custom function backends

feat(email): implement EmailSendJob class for robust email delivery with retry logic and MX record resolution

feat(mail): restructure mail module exports for simplified access to core and delivery functionalities

ts/mail/routing/classes.dns.manager.ts

@@ -1,7 +1,7 @@
 import * as plugins from '../../plugins.ts';
 import type { IEmailDomainConfig } from './interfaces.ts';
 import { logger } from '../../logger.ts';
-import type { DcRouter } from '../../classes.mailer.ts';
+import type { DcRouter } from '../../classes.dcrouter.ts';
 import type { StorageManager } from '../../storage/index.ts';
 
 /**