diff --git a/frappe/email/email_body.py b/frappe/email/email_body.py
index d545190c47..6d0c6f8f33 100755
--- a/frappe/email/email_body.py
+++ b/frappe/email/email_body.py
@@ -207,7 +207,11 @@ class EMail:
 
 	def set_in_reply_to(self, in_reply_to):
 		"""Used to send the Message-Id of a received email back as In-Reply-To"""
-		self.msg_root["In-Reply-To"] = in_reply_to
+		try:
+			self.msg_root["In-Reply-To"] = in_reply_to
+		except ValueError:
+			# in_reply_to may contain line feed characters, so ignore in that case
+			pass
 
 	def make(self):
 		"""build into msg_root"""