diff --git a/target_salesforce_v3/sinks.py b/target_salesforce_v3/sinks.py
index 6ff11f5..579e715 100644
--- a/target_salesforce_v3/sinks.py
+++ b/target_salesforce_v3/sinks.py
@@ -78,7 +78,9 @@ def preprocess_record(self, record: dict, context: dict):
             "Salutation": salutation,
             "Birthdate": birthdate,
             "OwnerId": record.get("owner_id"),
-            "HasOptedOutOfEmail": record.get("unsubscribed"),
+            "HasOptedOutOfEmail": record.get("unsubscribed")
+            if record.get("unsubscribed") is not None
+            else record.get("subscribe_status") == "unsubscribed",            
             "NumberOfEmployees": record.get("number_of_employees"),
             "Website": record.get("website"),
             "Industry": industry,
@@ -707,6 +709,10 @@ def preprocess_record(self, record, context):
 class FallbackSink(SalesforceV3Sink):
     endpoint = "sobjects/"
 
+    @property
+    def lookup_fields_dict(self):
+        return self.config.get("lookup_fields") or {}
+
     @property
     def name(self):
         return self.stream_name
@@ -756,29 +762,51 @@ def preprocess_record(self, record, context):
                     return {}
                 record["object_type"] = object_type
 
-                # Try to find object instance using email
-                email_fields = ["Email", "npe01__AlternateEmail__c", "npe01__HomeEmail__c", "npe01__Preferred_Email__c", "npe01__WorkEmail__c"]
-                email_values = [record.get(email_field) for email_field in email_fields if record.get(email_field)]
-                for email_to_check in email_values:
-                    # Escape special characters on email
-                    for char in ["+", "-"]:
-                        if char in email_to_check:
-                            email_to_check = email_to_check.replace(char, f"\{char}")
-
-                    query = "".join(["FIND {", email_to_check, "} ", f" IN ALL FIELDS RETURNING {object_type}(id)"])
-                    req = self.request_api("GET", "search/", params={"q": query})
-
-                    if req.json().get("searchRecords"):
-                        record["Id"] = req.json()["searchRecords"][0]["Id"]
-                        break
-                
+                # If lookup_fields dict exist in config use it to check if the record exists in Salesforce
+                object_lookup_field = self.lookup_fields_dict.get(object_type)
+                # check if the lookup field exists for the object
+                object_lookup_field = object_lookup_field if object_lookup_field in fields else None
+                # check if the record has a value for the lookup field
+                lookup_value = record.get(object_lookup_field)
+
+                req = None
+                # lookup for record with field from config
+                if object_lookup_field and lookup_value:
+                    query_fields = ",".join([field for field in fields.keys() if field in record] + ["Id"])
+                    query = f"SELECT {query_fields} FROM {object_type} WHERE {object_lookup_field} = '{lookup_value}'"
+                    req = self.request_api("GET", "queryAll", params={"q": query})
+                    req = req.json().get("records")
+                # lookup for record with email fields
+                else:
+                    # Try to find object instance using email
+                    email_fields = ["Email", "npe01__AlternateEmail__c", "npe01__HomeEmail__c", "npe01__Preferred_Email__c", "npe01__WorkEmail__c"]
+                    email_values = [record.get(email_field) for email_field in email_fields if record.get(email_field)]
+
+                    for email_to_check in email_values:
+                        # Escape special characters on email
+                        for char in ["+", "-"]:
+                            if char in email_to_check:
+                                email_to_check = email_to_check.replace(char, f"\{char}")
+
+                        query = "".join(["FIND {", email_to_check, "} ", f" IN ALL FIELDS RETURNING {object_type}(id)"])
+                        req = self.request_api("GET", "search/", params={"q": query})
+                        req = req.json().get("searchRecords")
+                        if req:
+                            break
+
+                # if record already exists add its Id for patching           
+                if req:
+                    existing_record = req[0]
+                    # if flag only_upsert_empty_fields is true, only send fields with currently empty values
+                    if self.config.get("only_upsert_empty_fields"):
+                        record = {k:v for k,v in record.items() if not existing_record.get(k)}
+                    record["Id"] = existing_record["Id"]
                 return record
             except TargetSalesforceQuotaExceededException as e:
                 return {"error": str(e)}
         else:
             return {"error": "Unprocessed record due to requests exceeded API rate limits"}
 
-    
 
     def upsert_record(self, record, context):
         # Not process records if target hit API rate limits