diff --git a/backend/services/email_client.py b/backend/services/email_client.py
index db17eb77a..8763a41aa 100644
--- a/backend/services/email_client.py
+++ b/backend/services/email_client.py
@@ -64,6 +64,10 @@ class SmtpConfig:
 
 def generate_oauth2_string(user: str, access_token: str) -> bytes:
     """Generates an OAuth2 string for IMAP/SMTP authentication."""
+    if "\x01" in user or "\x01" in access_token:
+        raise ValueError(
+            "OAuth2 authentication fields must not contain SASL delimiters"
+        )
     auth_string = f"user={user}\x01auth=Bearer {access_token}\x01\x01"
     return base64.b64encode(auth_string.encode("utf-8"))
 
diff --git a/backend/tests/test_email_client.py b/backend/tests/test_email_client.py
index ad8260c53..550e20c18 100644
--- a/backend/tests/test_email_client.py
+++ b/backend/tests/test_email_client.py
@@ -18,6 +18,21 @@ def test_generate_oauth2_string():
     assert b"auth=Bearer dummy_token" in decoded
 
 
+@pytest.mark.parametrize(
+    ("user", "access_token"),
+    [
+        ("victim@example.com\x01auth=Bearer attacker", "valid_token"),
+        ("victim@example.com", "valid_token\x01user=attacker@example.com"),
+    ],
+)
+def test_generate_oauth2_string_rejects_sasl_field_delimiters(user, access_token):
+    with pytest.raises(
+        ValueError,
+        match="OAuth2 authentication fields must not contain SASL delimiters",
+    ):
+        generate_oauth2_string(user, access_token)
+
+
 def test_build_email_message_sets_reply_headers():
     params = EmailMessageParams(
         to_address="test@example.com",
diff --git a/docs/research/email-authentication-xoauth2/README.md b/docs/research/email-authentication-xoauth2/README.md
new file mode 100644
index 000000000..d764ddf66
--- /dev/null
+++ b/docs/research/email-authentication-xoauth2/README.md
@@ -0,0 +1,54 @@
+# Email authentication — XOAUTH2 delimiter integrity
+
+This note grounds Naruon's SASL XOAUTH2 payload construction at
+`backend/services/email_client.py` and the hostile-input regression in
+`backend/tests/test_email_client.py`.
+
+## Protocol boundary
+
+RFC 7628 defines OAuth SASL key/value fields as being separated by the octet
+`%x01` (Control-A). Google's Gmail XOAUTH2 documentation uses the same wire
+shape for the initial client response: one `user` field, one
+`auth=Bearer ...` field, and a final empty field, each separated by Control-A.
+The delimiter is therefore protocol structure, not ordinary caller-controlled
+field data.
+
+Naruon's helper previously interpolated the supplied user identity and access
+token into that attribute stream before base64 encoding. A Control-A embedded
+inside either value created an additional protocol field boundary. Base64 does
+not remove that ambiguity; it only encodes the already-constructed octet
+sequence.
+
+## Decision
+
+`generate_oauth2_string()` rejects `\x01` in either the user identity or access
+token before the SASL response is constructed. The ordinary response format is
+unchanged. The function does not log credentials, repair malformed values,
+percent-encode the delimiter, introduce a fallback authentication mechanism, or
+broaden the allowed IMAP/SMTP destinations.
+
+The regression corpus covers delimiter injection through both caller-controlled
+fields and preserves the existing valid-payload test. This is a structural
+protocol validation rule rather than a keyword/security-score heuristic.
+
+## Claim boundary
+
+This change prevents caller data from introducing extra XOAUTH2 field
+separators at this construction boundary. It does not by itself claim complete
+OAuth, SASL, Gmail, IMAP, or SMTP security; token issuance, audience/scope,
+transport security, server policy, credential storage, TLS identity, egress
+allowlisting, and provider behavior remain separate controls.
+
+## References (APA 7)
+
+- Mills, W., Showalter, T., & Tschofenig, H. (2015). *A set of Simple
+  Authentication and Security Layer (SASL) mechanisms for OAuth* (RFC 7628).
+  RFC Editor. https://www.rfc-editor.org/rfc/rfc7628.html
+- Google. (n.d.). *OAuth 2.0 mechanism*. Google Workspace. Retrieved August 14,
+  2026, from https://developers.google.com/workspace/gmail/imap/xoauth2-protocol
+
+## Verification boundary
+
+The branch is not merge-ready merely because this note and the narrow fix
+exist. Current-head repository CI, security, coverage, independent review, and
+protected-branch gates remain authoritative.