chore(dovecot): 优化邮箱别名与默认文件夹创建

- 添加 `recipient_delimiter = +` 以支持加号别名。
- 为 Drafts、Sent、Trash、Archive、Spam 配置自动创建。

deploy/dovecot/dovecot.conf

@@ -11,6 +11,7 @@ ssl_key = </etc/ssl/private/ssl-cert-snakeoil.key
 
 mail_home = /var/mail/vhosts/%d/%n
 mail_max_userip_connections = 10
+recipient_delimiter = +
 
 plugin {
   quota = maildir:User quota
@@ -27,18 +28,23 @@ auth_policy_request_fields = protocol=imap username=user
 namespace inbox {
   inbox = yes
   mailbox Drafts {
+    auto = create
     special_use = \Drafts
   }
   mailbox Sent {
+    auto = create
     special_use = \Sent
   }
   mailbox Trash {
+    auto = create
     special_use = \Trash
   }
   mailbox Archive {
+    auto = create
     special_use = \Archive
   }
   mailbox Spam {
+    auto = create
     special_use = \Junk
   }
 }