diff --git a/gateway/platforms/email.py b/gateway/platforms/email.py
index f3e153c4e6a..f1a0ef07b6b 100644
--- a/gateway/platforms/email.py
+++ b/gateway/platforms/email.py
@@ -213,6 +213,7 @@ class EmailAdapter(BasePlatformAdapter):
 
         # Track message IDs we've already processed to avoid duplicates
         self._seen_uids: set = set()
+        self._seen_uids_max: int = 2000   # cap to prevent unbounded memory growth
         self._poll_task: Optional[asyncio.Task] = None
 
         # Map chat_id (sender email) -> last subject + message-id for threading
@@ -220,6 +221,26 @@ class EmailAdapter(BasePlatformAdapter):
 
         logger.info("[Email] Adapter initialized for %s", self._address)
 
+    def _trim_seen_uids(self) -> None:
+        """Keep only the most recent UIDs to prevent unbounded memory growth.
+
+        IMAP UIDs are monotonically increasing integers. When the set grows
+        beyond the cap, we keep only the highest half — old UIDs are safe to
+        drop because new messages always have higher UIDs and IMAP's UNSEEN
+        flag prevents re-delivery regardless.
+        """
+        if len(self._seen_uids) <= self._seen_uids_max:
+            return
+        try:
+            # UIDs are bytes like b'1234' — sort numerically and keep top half
+            sorted_uids = sorted(self._seen_uids, key=lambda u: int(u))
+            keep = self._seen_uids_max // 2
+            self._seen_uids = set(sorted_uids[-keep:])
+            logger.debug("[Email] Trimmed seen UIDs to %d entries", len(self._seen_uids))
+        except (ValueError, TypeError):
+            # Fallback: just clear old entries if sort fails
+            self._seen_uids = set(list(self._seen_uids)[-self._seen_uids_max // 2:])
+
     async def connect(self) -> bool:
         """Connect to the IMAP server and start polling for new messages."""
         try:
@@ -232,6 +253,8 @@ class EmailAdapter(BasePlatformAdapter):
             if status == "OK" and data and data[0]:
                 for uid in data[0].split():
                     self._seen_uids.add(uid)
+            # Keep only the most recent UIDs to prevent unbounded growth
+            self._trim_seen_uids()
             imap.logout()
             logger.info("[Email] IMAP connection test passed. %d existing messages skipped.", len(self._seen_uids))
         except Exception as e:
@@ -302,6 +325,9 @@ class EmailAdapter(BasePlatformAdapter):
                 if uid in self._seen_uids:
                     continue
                 self._seen_uids.add(uid)
+                # Trim periodically to prevent unbounded memory growth
+                if len(self._seen_uids) > self._seen_uids_max:
+                    self._trim_seen_uids()
 
                 status, msg_data = imap.uid("fetch", uid, "(RFC822)")
                 if status != "OK":