Undefined symbols for architecture i386: _OBJC_CLASS_$_SKPSMTPMessage", referenced from: error

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

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