我已经导入了用于从后台应用程序发送电子邮件的框架，即SKPSMTPMessage框架。有人可以建议为什么显示以下错误

Undefined symbols for architecture i386:

"_OBJC_CLASS_$_SKPSMTPMessage", referenced from:
  objc-class-ref in ConfirmController.o

"_kSKPSMTPPartContentTransferEncodingKey", referenced from:
  -[ConfirmController sendEmail] in ConfirmController.o

"_kSKPSMTPPartMessageKey", referenced from:
  -[ConfirmController sendEmail] in ConfirmController.o

"_kSKPSMTPPartContentTypeKey", referenced from:
  -[ConfirmController sendEmail] in ConfirmController.o

ld: symbol(s) not found for architecture i386
collect2: ld returned 1 exit status

更新 ：

答：只需将文件夹拖放到项目上，然后单击复制。那就是选择项目复选框和目标复选框。