diff --git a/changelog.d/18125.bugfix b/changelog.d/18125.bugfix
new file mode 100644
index 00000000000..0e6aa704fff
--- /dev/null
+++ b/changelog.d/18125.bugfix
@@ -0,0 +1 @@
+Fix a bug when updating a user 3pid with invalid returns 500 server error change to 400 with a message.
\ No newline at end of file
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 1f4264ad7e7..e96922c08d1 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -1579,7 +1579,10 @@ async def add_threepid(
         # for the presence of an email address during password reset was
         # case sensitive).
         if medium == "email":
-            address = canonicalise_email(address)
+            try:
+                address = canonicalise_email(address)
+            except ValueError as e:
+                raise SynapseError(400, str(e))
 
         await self.store.user_add_threepid(
             user_id, medium, address, validated_at, self.hs.get_clock().time_msec()
@@ -1610,7 +1613,10 @@ async def delete_local_threepid(
         """
         # 'Canonicalise' email addresses as per above
         if medium == "email":
-            address = canonicalise_email(address)
+            try:
+                address = canonicalise_email(address)
+            except ValueError as e:
+                raise SynapseError(400, str(e))
 
         await self.store.user_delete_threepid(user_id, medium, address)