diff --git a/Lib/email/contentmanager.py b/Lib/email/contentmanager.py
index b4f5830beada4a..506e99f6838468 100644
--- a/Lib/email/contentmanager.py
+++ b/Lib/email/contentmanager.py
@@ -1,3 +1,4 @@
+import sys
 import binascii
 import email.charset
 import email.message
@@ -142,13 +143,15 @@ def _encode_base64(data, max_line_length):
 
 
 def _encode_text(string, charset, cte, policy):
+    # max_line_length 0/None means no limit, ie: infinitely long.
+    maxlen = policy.max_line_length or sys.maxsize
     lines = string.encode(charset).splitlines()
     linesep = policy.linesep.encode('ascii')
     def embedded_body(lines): return linesep.join(lines) + linesep
     def normal_body(lines): return b'\n'.join(lines) + b'\n'
     if cte is None:
         # Use heuristics to decide on the "best" encoding.
-        if max((len(x) for x in lines), default=0) <= policy.max_line_length:
+        if max((len(x) for x in lines), default=0) <= maxlen:
             try:
                 return '7bit', normal_body(lines).decode('ascii')
             except UnicodeDecodeError:
@@ -157,7 +160,7 @@ def normal_body(lines): return b'\n'.join(lines) + b'\n'
                 return '8bit', normal_body(lines).decode('ascii', 'surrogateescape')
         sniff = embedded_body(lines[:10])
         sniff_qp = quoprimime.body_encode(sniff.decode('latin-1'),
-                                          policy.max_line_length)
+                                          maxlen)
         sniff_base64 = binascii.b2a_base64(sniff)
         # This is a little unfair to qp; it includes lineseps, base64 doesn't.
         if len(sniff_qp) > len(sniff_base64):
@@ -172,9 +175,9 @@ def normal_body(lines): return b'\n'.join(lines) + b'\n'
         data = normal_body(lines).decode('ascii', 'surrogateescape')
     elif cte == 'quoted-printable':
         data = quoprimime.body_encode(normal_body(lines).decode('latin-1'),
-                                      policy.max_line_length)
+                                      maxlen)
     elif cte == 'base64':
-        data = _encode_base64(embedded_body(lines), policy.max_line_length)
+        data = _encode_base64(embedded_body(lines), maxlen)
     else:
         raise ValueError("Unknown content transfer encoding {}".format(cte))
     return cte, data
diff --git a/Lib/test/test_email/test_message.py b/Lib/test/test_email/test_message.py
index 23c39775a8b2e5..6f7fb5f727e688 100644
--- a/Lib/test/test_email/test_message.py
+++ b/Lib/test/test_email/test_message.py
@@ -1004,6 +1004,34 @@ def test_folding_with_long_nospace_http_policy_1(self):
         parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
         self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
 
+    def test_no_wrapping_with_zero_max_line_length(self):
+        pol = policy.default.clone(max_line_length=0)
+        subj = "S" * 100
+        msg = EmailMessage(policy=pol)
+        msg["From"] = "a@ex.com"
+        msg["To"] = "b@ex.com"
+        msg["Subject"] = subj
+
+        raw = msg.as_bytes()
+        self.assertNotIn(b"\r\n ", raw, "Found fold indicator; wrapping not disabled")
+
+        parsed = message_from_bytes(raw, policy=policy.default)
+        self.assertEqual(parsed["Subject"], subj)
+
+    def test_no_wrapping_with_none_max_line_length(self):
+        pol = policy.default.clone(max_line_length=None)
+        subj = "S" * 100
+        body = "B" * 100
+        msg = EmailMessage(policy=pol)
+        msg["From"] = "a@ex.com"
+        msg["To"] = "b@ex.com"
+        msg["Subject"] = subj
+        msg.set_content(body)
+
+        parsed = message_from_bytes(msg.as_bytes(), policy=policy.default)
+        self.assertEqual(parsed["Subject"], subj)
+        self.assertEqual(parsed.get_body().get_content().rstrip('\n'), body)
+
     def test_invalid_header_names(self):
         invalid_headers = [
             ('Invalid Header', 'contains space'),
diff --git a/Misc/NEWS.d/next/Library/2025-06-10-18-02-29.gh-issue-135307.fXGrcK.rst b/Misc/NEWS.d/next/Library/2025-06-10-18-02-29.gh-issue-135307.fXGrcK.rst
new file mode 100644
index 00000000000000..2eed86d88d202b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-06-10-18-02-29.gh-issue-135307.fXGrcK.rst
@@ -0,0 +1,2 @@
+:mod:`email`: Ensure policy accepts unlimited line lengths by
+treating 0 or :const:`None` as :data:`sys.maxsize`.