diff --git a/src/memory/ingest/canonicalize/email.rs b/src/memory/ingest/canonicalize/email.rs
index 4ab2028..2bdcf01 100644
--- a/src/memory/ingest/canonicalize/email.rs
+++ b/src/memory/ingest/canonicalize/email.rs
@@ -17,6 +17,14 @@ use serde::{Deserialize, Serialize};
 use super::{email_clean, normalize_source_ref, CanonicalisedSource};
 use crate::memory::chunks::{Metadata, SourceKind};
 
+/// Returns the current UTC time as a timestamp default. Used by serde
+/// `#[serde(default = …)]` so an email payload missing the `sent_at` field
+/// doesn't reject the whole thread — it falls back to `now()`. Mirrors the
+/// same tolerance on `ChatMessage::timestamp`.
+fn chrono_now() -> DateTime<Utc> {
+    Utc::now()
+}
+
 /// One email in a thread.
 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct EmailMessage {
@@ -32,7 +40,11 @@ pub struct EmailMessage {
     /// Per-message subject; rendered as the `Subject:` header.
     pub subject: String,
     /// When the message was sent (epoch-ms integer or RFC 3339 string).
+    /// When absent from the payload, defaults to `Utc::now()` so that
+    /// producers that omit the field (version skew / third-party
+    /// integration) do not cause a hard rejection of the whole thread.
     #[serde(
+        default = "chrono_now",
         serialize_with = "chrono::serde::ts_milliseconds::serialize",
         deserialize_with = "super::deserialize_flexible_timestamp"
     )]
diff --git a/src/memory/ingest/canonicalize/email_tests.rs b/src/memory/ingest/canonicalize/email_tests.rs
index a7daffd..231b54e 100644
--- a/src/memory/ingest/canonicalize/email_tests.rs
+++ b/src/memory/ingest/canonicalize/email_tests.rs
@@ -204,3 +204,54 @@ fn headers_and_body_cannot_inject_email_boundaries() {
         .markdown
         .contains("\\---\nFrom: body-forgery@example.com"));
 }
+
+// ── Serde regression tests (sibling of #5169) ───────────────────────────────
+
+/// A payload with no `sent_at` field must deserialize gracefully (defaulting
+/// to `Utc::now()`) instead of failing with "missing field `sent_at`". This is
+/// the email-arm counterpart of the chat-arm fix in #5169.
+#[test]
+fn missing_sent_at_defaults_to_now() {
+    let json = r#"{
+        "from": "alice@example.com",
+        "subject": "hi",
+        "body": "hello"
+    }"#;
+    let msg: EmailMessage = serde_json::from_str(json).expect("missing sent_at should not fail");
+    let diff = Utc::now().signed_duration_since(msg.sent_at);
+    assert!(
+        diff.num_seconds().unsigned_abs() < 5,
+        "defaulted sent_at should be within ~5s of now, got {diff:?}"
+    );
+}
+
+/// A null `sent_at` should also fall back to the default.
+#[test]
+fn null_sent_at_defaults_to_now() {
+    let json = r#"{
+        "from": "alice@example.com",
+        "subject": "hi",
+        "sent_at": null,
+        "body": "hello"
+    }"#;
+    let msg: EmailMessage = serde_json::from_str(json).expect("null sent_at should not fail");
+    let diff = Utc::now().signed_duration_since(msg.sent_at);
+    assert!(
+        diff.num_seconds().unsigned_abs() < 5,
+        "defaulted sent_at should be within ~5s of now, got {diff:?}"
+    );
+}
+
+/// Explicit epoch-ms `sent_at` still parses exactly (guards against the default
+/// masking a real value).
+#[test]
+fn explicit_sent_at_still_parses() {
+    let json = r#"{
+        "from": "alice@example.com",
+        "subject": "hi",
+        "sent_at": 1700000000000,
+        "body": "hello"
+    }"#;
+    let msg: EmailMessage = serde_json::from_str(json).expect("epoch-ms sent_at should parse");
+    assert_eq!(msg.sent_at.timestamp_millis(), 1_700_000_000_000);
+}