urlize: "@a@b" isn't a valid e-mail, don't mailto:ise it

CHANGES.rst

@@ -8,6 +8,7 @@ Unreleased
 -   Use modern packaging metadata with ``pyproject.toml`` instead of ``setup.cfg``.
     :pr:`1793`
 -   Use ``flit_core`` instead of ``setuptools`` as build backend.
+-   Don't ``mailto:``-``urlize()`` addresses in the form ``@a@b``. :pr:`1870`
 
 
 Version 3.1.3

src/jinja2/utils.py

@@ -324,6 +324,7 @@ def trim_url(x: str) -> str:
         elif (
             "@" in middle
             and not middle.startswith("www.")
+            and not middle.startswith("@")
             and ":" not in middle
             and _email_re.match(middle)
         ):

tests/test_utils.py

@@ -141,6 +141,14 @@ def test_escape_urlize_target(self):
             "http://example.org</a>"
         )
 
+    def test_urlize_mail_mastodon(self):
+        fr = "[email protected]\n@[email protected]\n"
+        to = (
+            '<a href="mailto:[email protected]">'
+            "[email protected]</a>\n@[email protected]\n"
+        )
+        assert urlize(fr) == to
+
 
 class TestLoremIpsum:
     def test_lorem_ipsum_markup(self):