我已经导入框架从应用程序发送电子邮件在后台即SKPSMTPMessage框架。有人能建议为什么下面的错误显示

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

框架的来源:-

锁定MFMailComposeViewController中的字段

更新:

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