diff --git a/frappe/email/doctype/email_account/email_account.py b/frappe/email/doctype/email_account/email_account.py
index ba2f178424..bf1bd4fdd9 100644
--- a/frappe/email/doctype/email_account/email_account.py
+++ b/frappe/email/doctype/email_account/email_account.py
@@ -192,18 +192,27 @@ class EmailAccount(Document):
 							parent = frappe.get_doc(parent.reference_doctype,
 								parent.reference_name)
 
-		if not parent and self.append_to and subject_field and sender_field:
-			# try and match by subject and sender
-			# if sent by same sender with same subject,
-			# append it to old coversation
-
-			subject = re.sub("Re[^:]*:\s*", "", email.subject)
-
-			parent = frappe.db.get_all(self.append_to, filters={
-				sender_field: email.from_email,
-				subject_field: ("like", "%{0}%".format(subject)),
-				"creation": (">", (get_datetime() - relativedelta(days=10)).strftime(DATE_FORMAT))
-			}, fields="name")
+		if not parent and self.append_to and sender_field:
+			if subject_field:
+				# try and match by subject and sender
+				# if sent by same sender with same subject,
+				# append it to old coversation
+
+				subject = re.sub("Re[^:]*:\s*", "", email.subject)
+
+				parent = frappe.db.get_all(self.append_to, filters={
+					sender_field: email.from_email,
+					subject_field: ("like", "%{0}%".format(subject)),
+					"creation": (">", (get_datetime() - relativedelta(days=10)).strftime(DATE_FORMAT))
+				}, fields="name")
+
+			else:
+				# try and match by sender only
+				# as there is no subject field, it implies that threading isn't by subject, but by sender only
+
+				parent = frappe.db.get_all(self.append_to, filters={
+					sender_field: email.from_email,
+				}, fields="name")
 
 			if parent:
 				parent = frappe.get_doc(self.append_to, parent[0].name)