diff --git a/src/cryptography/hazmat/primitives/serialization/pkcs7.py b/src/cryptography/hazmat/primitives/serialization/pkcs7.py
index 7b8ab300fecb..593c9b159db3 100644
--- a/src/cryptography/hazmat/primitives/serialization/pkcs7.py
+++ b/src/cryptography/hazmat/primitives/serialization/pkcs7.py
@@ -5,6 +5,7 @@
 import email.base64mime
 import email.generator
 import email.message
+import email.policy
 import io
 import typing
 
@@ -176,7 +177,9 @@ def sign(
         return rust_pkcs7.sign_and_serialize(self, encoding, options)
 
 
-def _smime_encode(data: bytes, signature: bytes, micalg: str) -> bytes:
+def _smime_encode(
+    data: bytes, signature: bytes, micalg: str, text_mode: bool
+) -> bytes:
     # This function works pretty hard to replicate what OpenSSL does
     # precisely. For good and for ill.
 
@@ -191,9 +194,10 @@ def _smime_encode(data: bytes, signature: bytes, micalg: str) -> bytes:
 
     m.preamble = "This is an S/MIME signed message\n"
 
-    msg_part = email.message.MIMEPart()
+    msg_part = OpenSSLMimePart()
     msg_part.set_payload(data)
-    msg_part.add_header("Content-Type", "text/plain")
+    if text_mode:
+        msg_part.add_header("Content-Type", "text/plain")
     m.attach(msg_part)
 
     sig_part = email.message.MIMEPart()
@@ -212,7 +216,18 @@ def _smime_encode(data: bytes, signature: bytes, micalg: str) -> bytes:
 
     fp = io.BytesIO()
     g = email.generator.BytesGenerator(
-        fp, maxheaderlen=0, mangle_from_=False, policy=m.policy
+        fp,
+        maxheaderlen=0,
+        mangle_from_=False,
+        policy=m.policy.clone(linesep="\r\n"),
     )
     g.flatten(m)
     return fp.getvalue()
+
+
+class OpenSSLMimePart(email.message.MIMEPart):
+    # A MIMEPart subclass that replicates OpenSSL's behavior of not including
+    # a newline if there are no headers.
+    def _write_headers(self, generator) -> None:
+        if list(self.raw_items()):
+            generator._write_headers(self)
diff --git a/src/rust/src/pkcs7.rs b/src/rust/src/pkcs7.rs
index d760776564e3..99488e13dc15 100644
--- a/src/rust/src/pkcs7.rs
+++ b/src/rust/src/pkcs7.rs
@@ -135,14 +135,15 @@ fn sign_and_serialize<'p>(
         .getattr(crate::intern!(py, "PKCS7Options"))?;
 
     let raw_data = builder.getattr(crate::intern!(py, "_data"))?.extract()?;
-    let data = if options.contains(pkcs7_options.getattr(crate::intern!(py, "Binary"))?)? {
-        Cow::Borrowed(raw_data)
-    } else {
-        smime_canonicalize(
-            raw_data,
-            options.contains(pkcs7_options.getattr(crate::intern!(py, "Text"))?)?,
-        )
-    };
+    let (data_with_header, data_without_header) =
+        if options.contains(pkcs7_options.getattr(crate::intern!(py, "Binary"))?)? {
+            (Cow::Borrowed(raw_data), Cow::Borrowed(raw_data))
+        } else {
+            smime_canonicalize(
+                raw_data,
+                options.contains(pkcs7_options.getattr(crate::intern!(py, "Text"))?)?,
+            )
+        };
 
     let content_type_bytes = asn1::write_single(&PKCS7_DATA_OID)?;
     let signing_time_bytes = asn1::write_single(&x509::certificate::time_from_chrono(
@@ -179,7 +180,7 @@ fn sign_and_serialize<'p>(
         {
             (
                 None,
-                x509::sign::sign_data(py, py_private_key, py_hash_alg, &data)?,
+                x509::sign::sign_data(py, py_private_key, py_hash_alg, &data_with_header)?,
             )
         } else {
             let mut authenticated_attrs = vec![];
@@ -197,7 +198,8 @@ fn sign_and_serialize<'p>(
                 ])),
             });
 
-            let digest = asn1::write_single(&x509::ocsp::hash_data(py, py_hash_alg, &data)?)?;
+            let digest =
+                asn1::write_single(&x509::ocsp::hash_data(py, py_hash_alg, &data_with_header)?)?;
             // Gross hack: copy to PyBytes to extend the lifetime to 'p
             let digest_bytes = pyo3::types::PyBytes::new(py, &digest);
             authenticated_attrs.push(x509::csr::Attribute {
@@ -263,7 +265,7 @@ fn sign_and_serialize<'p>(
         if options.contains(pkcs7_options.getattr(crate::intern!(py, "DetachedSignature"))?)? {
             None
         } else {
-            data_tlv_bytes = asn1::write_single(&data.deref())?;
+            data_tlv_bytes = asn1::write_single(&data_with_header.deref())?;
             Some(asn1::parse_single(&data_tlv_bytes).unwrap())
         };
 
@@ -305,9 +307,10 @@ fn sign_and_serialize<'p>(
             .import("cryptography.hazmat.primitives.serialization.pkcs7")?
             .getattr(crate::intern!(py, "_smime_encode"))?
             .call1((
-                pyo3::types::PyBytes::new(py, &data),
+                pyo3::types::PyBytes::new(py, &data_without_header),
                 pyo3::types::PyBytes::new(py, &content_info_bytes),
                 mic_algs,
+                options.contains(pkcs7_options.getattr(crate::intern!(py, "Text"))?)?,
             ))?
             .extract()?)
     } else {
@@ -316,28 +319,37 @@ fn sign_and_serialize<'p>(
     }
 }
 
-fn smime_canonicalize(data: &[u8], text_mode: bool) -> Cow<'_, [u8]> {
-    let mut new_data = vec![];
+fn smime_canonicalize(data: &[u8], text_mode: bool) -> (Cow<'_, [u8]>, Cow<'_, [u8]>) {
+    let mut new_data_with_header = vec![];
+    let mut new_data_without_header = vec![];
     if text_mode {
-        new_data.extend_from_slice(b"Content-Type: text/plain\r\n\r\n");
+        new_data_with_header.extend_from_slice(b"Content-Type: text/plain\r\n\r\n");
     }
 
     let mut last_idx = 0;
     for (i, c) in data.iter().copied().enumerate() {
         if c == b'\n' && (i == 0 || data[i - 1] != b'\r') {
-            new_data.extend_from_slice(&data[last_idx..i]);
-            new_data.push(b'\r');
-            new_data.push(b'\n');
+            new_data_with_header.extend_from_slice(&data[last_idx..i]);
+            new_data_with_header.push(b'\r');
+            new_data_with_header.push(b'\n');
+
+            new_data_without_header.extend_from_slice(&data[last_idx..i]);
+            new_data_without_header.push(b'\r');
+            new_data_without_header.push(b'\n');
             last_idx = i + 1;
         }
     }
     // If there's stuff in new_data, that means we need to copy the rest of
     // data over.
-    if !new_data.is_empty() {
-        new_data.extend_from_slice(&data[last_idx..]);
-        Cow::Owned(new_data)
+    if !new_data_with_header.is_empty() {
+        new_data_with_header.extend_from_slice(&data[last_idx..]);
+        new_data_without_header.extend_from_slice(&data[last_idx..]);
+        (
+            Cow::Owned(new_data_with_header),
+            Cow::Owned(new_data_without_header),
+        )
     } else {
-        Cow::Borrowed(data)
+        (Cow::Borrowed(data), Cow::Borrowed(data))
     }
 }
 
@@ -358,27 +370,60 @@ mod tests {
 
     #[test]
     fn test_smime_canonicalize() {
-        for (input, text_mode, expected, expected_is_borrowed) in [
+        for (
+            input,
+            text_mode,
+            expected_with_header,
+            expected_without_header,
+            expected_is_borrowed,
+        ) in [
             // Values with text_mode=false
-            (b"" as &[u8], false, b"" as &[u8], true),
-            (b"\n", false, b"\r\n", false),
-            (b"abc", false, b"abc", true),
-            (b"abc\r\ndef\n", false, b"abc\r\ndef\r\n", false),
-            (b"abc\r\n", false, b"abc\r\n", true),
-            (b"abc\ndef\n", false, b"abc\r\ndef\r\n", false),
+            (b"" as &[u8], false, b"" as &[u8], b"" as &[u8], true),
+            (b"\n", false, b"\r\n", b"\r\n", false),
+            (b"abc", false, b"abc", b"abc", true),
+            (
+                b"abc\r\ndef\n",
+                false,
+                b"abc\r\ndef\r\n",
+                b"abc\r\ndef\r\n",
+                false,
+            ),
+            (b"abc\r\n", false, b"abc\r\n", b"abc\r\n", true),
+            (
+                b"abc\ndef\n",
+                false,
+                b"abc\r\ndef\r\n",
+                b"abc\r\ndef\r\n",
+                false,
+            ),
             // Values with text_mode=true
-            (b"", true, b"Content-Type: text/plain\r\n\r\n", false),
-            (b"abc", true, b"Content-Type: text/plain\r\n\r\nabc", false),
+            (b"", true, b"Content-Type: text/plain\r\n\r\n", b"", false),
+            (
+                b"abc",
+                true,
+                b"Content-Type: text/plain\r\n\r\nabc",
+                b"abc",
+                false,
+            ),
             (
                 b"abc\n",
                 true,
                 b"Content-Type: text/plain\r\n\r\nabc\r\n",
+                b"abc\r\n",
                 false,
             ),
         ] {
-            let result = smime_canonicalize(input, text_mode);
-            assert_eq!(result.deref(), expected);
-            assert_eq!(matches!(result, Cow::Borrowed(_)), expected_is_borrowed);
+            let (result_with_header, result_without_header) = smime_canonicalize(input, text_mode);
+            assert_eq!(result_with_header.deref(), expected_with_header);
+            assert_eq!(result_without_header.deref(), expected_without_header);
+            assert_eq!(
+                matches!(result_with_header, Cow::Borrowed(_)),
+                expected_is_borrowed
+            );
+            assert_eq!(
+                matches!(result_without_header, Cow::Borrowed(_)),
+                expected_is_borrowed
+            );
         }
     }
 }
diff --git a/tests/hazmat/primitives/test_pkcs7.py b/tests/hazmat/primitives/test_pkcs7.py
index ebb8dc0a9baa..d879563e17d9 100644
--- a/tests/hazmat/primitives/test_pkcs7.py
+++ b/tests/hazmat/primitives/test_pkcs7.py
@@ -3,6 +3,7 @@
 # for complete details.
 
 
+import email.parser
 import os
 import typing
 
@@ -289,6 +290,7 @@ def test_smime_sign_detached(self, backend):
 
         sig = builder.sign(serialization.Encoding.SMIME, options)
         sig_binary = builder.sign(serialization.Encoding.DER, options)
+        assert b"text/plain" not in sig
         # We don't have a generic ASN.1 parser available to us so we instead
         # will assert on specific byte sequences being present based on the
         # parameters chosen above.
@@ -298,8 +300,17 @@ def test_smime_sign_detached(self, backend):
         # as a separate section before the PKCS7 data. So we should expect to
         # have data in sig but not in sig_binary
         assert data in sig
+        # Parse the message to get the signed data, which is the
+        # first payload in the message
+        message = email.parser.BytesParser().parsebytes(sig)
+        signed_data = message.get_payload()[0].get_payload().encode()
         _pkcs7_verify(
-            serialization.Encoding.SMIME, sig, data, [cert], options, backend
+            serialization.Encoding.SMIME,
+            sig,
+            signed_data,
+            [cert],
+            options,
+            backend,
         )
         assert data not in sig_binary
         _pkcs7_verify(
@@ -492,10 +503,14 @@ def test_sign_text(self, backend):
         # The text option adds text/plain headers to the S/MIME message
         # These headers are only relevant in SMIME mode, not binary, which is
         # just the PKCS7 structure itself.
-        assert b"text/plain" in sig_pem
-        # When passing the Text option the header is prepended so the actual
-        # signed data is this.
-        signed_data = b"Content-Type: text/plain\r\n\r\nhello world"
+        assert sig_pem.count(b"text/plain") == 1
+        assert b"Content-Type: text/plain\r\n\r\nhello world\r\n" in sig_pem
+        # Parse the message to get the signed data, which is the
+        # first payload in the message
+        message = email.parser.BytesParser().parsebytes(sig_pem)
+        signed_data = message.get_payload()[0].as_bytes(
+            policy=message.policy.clone(linesep="\r\n")
+        )
         _pkcs7_verify(
             serialization.Encoding.SMIME,
             sig_pem,