diff --git a/api/libs/helper.py b/api/libs/helper.py
index 7066f9eab45..752342bfad6 100644
--- a/api/libs/helper.py
+++ b/api/libs/helper.py
@@ -221,8 +221,11 @@ def current_timestamp() -> int:
 def email(email):
     # Define a regex pattern for email addresses
     pattern = r"^[\w\.!#$%&'*+\-/=?^_`{|}~]+@([\w-]+\.)+[\w-]{2,}$"
-    # Check if the email matches the pattern
-    if re.match(pattern, email) is not None:
+    # Use re.fullmatch instead of re.match to reject trailing newlines.
+    # In Python, '$' matches at end-of-string OR just before a trailing newline,
+    # so re.match accepts "user@example.com\n". re.fullmatch requires the entire
+    # string to match, closing the mail header-injection vector. (#39234)
+    if re.fullmatch(pattern, email) is not None:
         return email
 
     error = f"{email} is not a valid email."
diff --git a/api/tests/unit_tests/libs/test_helper.py b/api/tests/unit_tests/libs/test_helper.py
index 1a93dbbca13..dbc2cd6cba8 100644
--- a/api/tests/unit_tests/libs/test_helper.py
+++ b/api/tests/unit_tests/libs/test_helper.py
@@ -2,7 +2,7 @@ from datetime import datetime
 
 import pytest
 
-from libs.helper import OptionalTimestampField, escape_like_pattern, extract_tenant_id
+from libs.helper import OptionalTimestampField, email, escape_like_pattern, extract_tenant_id
 from models.account import Account
 from models.model import EndUser
 
@@ -126,3 +126,30 @@ class TestEscapeLikePattern:
         result = escape_like_pattern("test\\%_value")
         # Should be: test\\\%\_value
         assert result == "test\\\\\\%\\_value"
+
+
+class TestEmailValidator:
+    """Tests for the email() validator — regression for #39234."""
+
+    def test_valid_email_accepted(self):
+        assert email("user@example.com") == "user@example.com"
+
+    def test_trailing_newline_rejected(self):
+        with pytest.raises(ValueError, match="not a valid email"):
+            email("user@example.com\n")
+
+    def test_trailing_carriage_return_newline_rejected(self):
+        with pytest.raises(ValueError, match="not a valid email"):
+            email("user@example.com\r\n")
+
+    def test_multiple_newlines_rejected(self):
+        with pytest.raises(ValueError, match="not a valid email"):
+            email("user@example.com\n\n")
+
+    def test_empty_string_rejected(self):
+        with pytest.raises(ValueError, match="not a valid email"):
+            email("")
+
+    def test_invalid_email_rejected(self):
+        with pytest.raises(ValueError, match="not a valid email"):
+            email("not-an-email")