diff --git a/components/email/email-viewer.tsx b/components/email/email-viewer.tsx
index 5f7e5668..04bf8c68 100644
--- a/components/email/email-viewer.tsx
+++ b/components/email/email-viewer.tsx
@@ -1545,7 +1545,7 @@ export function EmailViewer({
 
         if (detection.type === 'enveloped-data') {
           // Encrypted message
-            const { keyRecords, unlockedDecryptionKeys } = smimeStore;
+            const { keyRecords, unlockedDecryptionKeys, unlockedLegacyDecryptionKeys } = smimeStore;
             smimeDebug('[S/MIME] decrypt attempt:', { keyRecordCount: keyRecords.length, unlockedKeyCount: unlockedDecryptionKeys.size, keyRecordIds: keyRecords.map(k => k.id) });
 
           // Short-circuit: no keys imported at all
@@ -1569,6 +1569,7 @@ export function EmailViewer({
                   cmsBytes: candidate.normalized,
                   keyRecords,
                     unlockedKeys: unlockedDecryptionKeys,
+                    legacyUnlockedKeys: unlockedLegacyDecryptionKeys,
                 });
                 smimeDebug('[S/MIME] decrypt success with candidate:', {
                   source: candidate.source,
@@ -1804,6 +1805,7 @@ export function EmailViewer({
     smimeStore.autoImportSignerCerts,
     smimeStore.keyRecords,
     smimeStore.unlockedDecryptionKeys,
+    smimeStore.unlockedLegacyDecryptionKeys,
     smimeStore,
   ]);
 
diff --git a/lib/smime/crypto-engine.ts b/lib/smime/crypto-engine.ts
index 1cb7761b..00ff218f 100644
--- a/lib/smime/crypto-engine.ts
+++ b/lib/smime/crypto-engine.ts
@@ -274,6 +274,12 @@ export function getLinerCryptoEngine(): pkijs.CryptoEngine {
   return linerEngine!;
 }
 
+/** Get the webcrypto-liner Crypto instance (for importKey with legacy algorithms). */
+export function getLinerCrypto(): Crypto {
+  ensureLiner();
+  return linerCryptoInstance!;
+}
+
 /**
  * Run an async operation with the global PKI.js engine set to webcrypto-liner,
  * then restore the previous engine afterwards.
diff --git a/lib/smime/pkcs12-import.ts b/lib/smime/pkcs12-import.ts
index 6e57b629..3be315bc 100644
--- a/lib/smime/pkcs12-import.ts
+++ b/lib/smime/pkcs12-import.ts
@@ -5,7 +5,7 @@ import {
   classifyCapabilities,
 } from './certificate-utils';
 import type { SmimeKeyRecord, Pkcs12ImportResult } from './types';
-import { withLinerEngine } from './crypto-engine';
+import { withLinerEngine, getLinerCrypto } from './crypto-engine';
 
 const KDF_ITERATIONS = 600_000;
 const AES_KEY_LENGTH = 256;
@@ -217,6 +217,8 @@ async function encryptPrivateKey(
 export interface UnlockedKeyPair {
   signingKey: CryptoKey;
   decryptionKey?: CryptoKey;
+  /** Key imported via webcrypto-liner as RSAES-PKCS1-v1_5 for legacy S/MIME (3DES) messages */
+  legacyDecryptionKey?: CryptoKey;
 }
 
 /** Decrypt stored PKCS#8 bytes and import as non-extractable CryptoKeys for signing and decryption. */
@@ -257,7 +259,22 @@ export async function unlockPrivateKey(
   } catch {
     // Key may only support decryption (key-encipherment-only cert)
     const decryptionKey = await crypto.subtle.importKey('pkcs8', pkcs8Bytes, decryptAlg, false, decryptUsages);
-    return { signingKey: decryptionKey, decryptionKey };
+    let legacyDecryptionKey: CryptoKey | undefined;
+    if (!isEcdsa) {
+      try {
+        const linerCrypto = getLinerCrypto();
+        legacyDecryptionKey = await linerCrypto.subtle.importKey(
+          'pkcs8',
+          pkcs8Bytes,
+          { name: 'RSAES-PKCS1-v1_5' },
+          false,
+          ['decrypt'],
+        );
+      } catch {
+        // webcrypto-liner may not be available
+      }
+    }
+    return { signingKey: decryptionKey, decryptionKey, legacyDecryptionKey };
   }
 
   // Also import for decryption (separate CryptoKey handle required by Web Crypto)
@@ -268,7 +285,25 @@ export async function unlockPrivateKey(
     // Key may only support signing (digitalSignature-only cert)
   }
 
-  return { signingKey, decryptionKey };
+  // Import a legacy decryption key via webcrypto-liner for RSAES-PKCS1-v1_5 key transport
+  // (used by older S/MIME messages encrypted with 3DES, RC2, etc.)
+  let legacyDecryptionKey: CryptoKey | undefined;
+  if (!isEcdsa) {
+    try {
+      const linerCrypto = getLinerCrypto();
+      legacyDecryptionKey = await linerCrypto.subtle.importKey(
+        'pkcs8',
+        pkcs8Bytes,
+        { name: 'RSAES-PKCS1-v1_5' },
+        false,
+        ['decrypt'],
+      );
+    } catch {
+      // webcrypto-liner may not be available or key format unsupported
+    }
+  }
+
+  return { signingKey, decryptionKey, legacyDecryptionKey };
 }
 
 /** Get decrypted PKCS#8 bytes (for export flow). */
diff --git a/lib/smime/smime-decrypt.ts b/lib/smime/smime-decrypt.ts
index 93a8279b..3a641687 100644
--- a/lib/smime/smime-decrypt.ts
+++ b/lib/smime/smime-decrypt.ts
@@ -15,8 +15,10 @@ export interface DecryptionInput {
   cmsBytes: ArrayBuffer;
   /** All imported key records to try matching against */
   keyRecords: SmimeKeyRecord[];
-  /** Unlocked CryptoKey map: keyRecordId → CryptoKey */
+  /** Unlocked CryptoKey map: keyRecordId → CryptoKey (RSA-OAEP) */
   unlockedKeys: Map<string, CryptoKey>;
+  /** Unlocked legacy CryptoKey map: keyRecordId → CryptoKey (RSAES-PKCS1-v1_5 via webcrypto-liner) */
+  legacyUnlockedKeys?: Map<string, CryptoKey>;
 }
 
 export interface DecryptionResult {
@@ -34,7 +36,7 @@ export interface DecryptionResult {
  * @throws Error if no matching key is found, key is locked, or decryption fails
  */
 export async function smimeDecrypt(input: DecryptionInput): Promise<DecryptionResult> {
-  const { cmsBytes, keyRecords, unlockedKeys } = input;
+  const { cmsBytes, keyRecords, unlockedKeys, legacyUnlockedKeys } = input;
 
   // Parse the CMS ContentInfo wrapper
   const contentInfo = parseContentInfo(cmsBytes);
@@ -51,6 +53,19 @@ export async function smimeDecrypt(input: DecryptionInput): Promise<DecryptionRe
   for (const { keyRecord, recipientIndex } of matchedRecords) {
     const privateKey = unlockedKeys.get(keyRecord.id);
     if (!privateKey) {
+      // Try legacy key (RSAES-PKCS1-v1_5) if no RSA-OAEP key
+      const legacyKey = legacyUnlockedKeys?.get(keyRecord.id);
+      if (legacyKey) {
+        try {
+          const decrypted = await decryptWithKey(envelopedData, recipientIndex, legacyKey, keyRecord);
+          return {
+            mimeBytes: new Uint8Array(decrypted),
+            keyRecordId: keyRecord.id,
+          };
+        } catch {
+          continue;
+        }
+      }
       continue; // Key exists but isn't unlocked — skip, caller should unlock first
     }
 
@@ -61,15 +76,28 @@ export async function smimeDecrypt(input: DecryptionInput): Promise<DecryptionRe
         keyRecordId: keyRecord.id,
       };
     } catch {
-      // This key didn't work, try the next one
+      // RSA-OAEP key didn't work, try legacy RSAES-PKCS1-v1_5 key
+      const legacyKey = legacyUnlockedKeys?.get(keyRecord.id);
+      if (legacyKey) {
+        try {
+          const decrypted = await decryptWithKey(envelopedData, recipientIndex, legacyKey, keyRecord);
+          return {
+            mimeBytes: new Uint8Array(decrypted),
+            keyRecordId: keyRecord.id,
+          };
+        } catch {
+          // Legacy key also didn't work, try the next record
+        }
+      }
       continue;
     }
   }
 
   // Check if we had matching records but none were unlocked
-  const hasLockedMatch = matchedRecords.some(m => !unlockedKeys.has(m.keyRecord.id));
+  const isUnlocked = (id: string) => unlockedKeys.has(id) || (legacyUnlockedKeys?.has(id) ?? false);
+  const hasLockedMatch = matchedRecords.some(m => !isUnlocked(m.keyRecord.id));
   if (hasLockedMatch) {
-    const lockedRecord = matchedRecords.find(m => !unlockedKeys.has(m.keyRecord.id))!;
+    const lockedRecord = matchedRecords.find(m => !isUnlocked(m.keyRecord.id))!;
     throw new SmimeKeyLockedError(
       'S/MIME key is locked. Unlock it to decrypt this message.',
       lockedRecord.keyRecord.id,
diff --git a/stores/smime-store.ts b/stores/smime-store.ts
index 3301dcdc..25b7594c 100644
--- a/stores/smime-store.ts
+++ b/stores/smime-store.ts
@@ -91,10 +91,12 @@ function clearRememberedUnlocks(): void {
 async function restoreRememberedKeys(keyRecords: SmimeKeyRecord[]): Promise<{
   unlockedKeys: Map<string, CryptoKey>;
   unlockedDecryptionKeys: Map<string, CryptoKey>;
+  unlockedLegacyDecryptionKeys: Map<string, CryptoKey>;
 }> {
   const rememberedUnlocks = readRememberedUnlocks();
   const unlockedKeys = new Map<string, CryptoKey>();
   const unlockedDecryptionKeys = new Map<string, CryptoKey>();
+  const unlockedLegacyDecryptionKeys = new Map<string, CryptoKey>();
   let removedStaleEntries = false;
 
   for (const record of keyRecords) {
@@ -104,11 +106,14 @@ async function restoreRememberedKeys(keyRecords: SmimeKeyRecord[]): Promise<{
     }
 
     try {
-      const { signingKey, decryptionKey } = await unlockPrivateKey(record, passphrase);
+      const { signingKey, decryptionKey, legacyDecryptionKey } = await unlockPrivateKey(record, passphrase);
       unlockedKeys.set(record.id, signingKey);
       if (decryptionKey) {
         unlockedDecryptionKeys.set(record.id, decryptionKey);
       }
+      if (legacyDecryptionKey) {
+        unlockedLegacyDecryptionKeys.set(record.id, legacyDecryptionKey);
+      }
     } catch {
       delete rememberedUnlocks[record.id];
       removedStaleEntries = true;
@@ -119,7 +124,7 @@ async function restoreRememberedKeys(keyRecords: SmimeKeyRecord[]): Promise<{
     writeRememberedUnlocks(rememberedUnlocks);
   }
 
-  return { unlockedKeys, unlockedDecryptionKeys };
+  return { unlockedKeys, unlockedDecryptionKeys, unlockedLegacyDecryptionKeys };
 }
 
 interface SmimePersistedState {
@@ -146,6 +151,7 @@ interface SmimeStore extends SmimePersistedState {
   // Runtime only — never persisted
   unlockedKeys: Map<string, CryptoKey>;
   unlockedDecryptionKeys: Map<string, CryptoKey>;
+  unlockedLegacyDecryptionKeys: Map<string, CryptoKey>;
   isLoading: boolean;
   error: string | null;
 
@@ -189,6 +195,7 @@ export const useSmimeStore = create<SmimeStore>()(
       publicCerts: [],
       unlockedKeys: new Map(),
       unlockedDecryptionKeys: new Map(),
+      unlockedLegacyDecryptionKeys: new Map(),
       isLoading: false,
       error: null,
 
@@ -231,6 +238,10 @@ export const useSmimeStore = create<SmimeStore>()(
                 ...state.unlockedDecryptionKeys,
                 ...restoredKeys.unlockedDecryptionKeys,
               ]),
+              unlockedLegacyDecryptionKeys: new Map([
+                ...state.unlockedLegacyDecryptionKeys,
+                ...restoredKeys.unlockedLegacyDecryptionKeys,
+              ]),
               isLoading: false,
             }));
             return;
@@ -332,6 +343,8 @@ export const useSmimeStore = create<SmimeStore>()(
           unlockedKeys.delete(id);
           const unlockedDecryptionKeys = new Map(state.unlockedDecryptionKeys);
           unlockedDecryptionKeys.delete(id);
+          const unlockedLegacyDecryptionKeys = new Map(state.unlockedLegacyDecryptionKeys);
+          unlockedLegacyDecryptionKeys.delete(id);
           // Remove any identity bindings pointing to this key
           const bindings = { ...state.identityKeyBindings };
           for (const [identityId, keyId] of Object.entries(bindings)) {
@@ -346,6 +359,7 @@ export const useSmimeStore = create<SmimeStore>()(
             keyRecords: state.keyRecords.filter((k) => k.id !== id),
             unlockedKeys,
             unlockedDecryptionKeys,
+            unlockedLegacyDecryptionKeys,
             identityKeyBindings: bindings,
             accountPreferences,
           };
@@ -363,7 +377,7 @@ export const useSmimeStore = create<SmimeStore>()(
         const record = get().keyRecords.find((k) => k.id === id);
         if (!record) throw new Error('Key record not found');
 
-        const { signingKey, decryptionKey } = await unlockPrivateKey(record, passphrase);
+        const { signingKey, decryptionKey, legacyDecryptionKey } = await unlockPrivateKey(record, passphrase);
         if (get().rememberUnlockedKeys) {
           rememberUnlockedKey(id, passphrase);
         }
@@ -374,7 +388,11 @@ export const useSmimeStore = create<SmimeStore>()(
           if (decryptionKey) {
             unlockedDecryptionKeys.set(id, decryptionKey);
           }
-          return { unlockedKeys, unlockedDecryptionKeys };
+          const unlockedLegacyDecryptionKeys = new Map(state.unlockedLegacyDecryptionKeys);
+          if (legacyDecryptionKey) {
+            unlockedLegacyDecryptionKeys.set(id, legacyDecryptionKey);
+          }
+          return { unlockedKeys, unlockedDecryptionKeys, unlockedLegacyDecryptionKeys };
         });
       },
 
@@ -385,13 +403,15 @@ export const useSmimeStore = create<SmimeStore>()(
           unlockedKeys.delete(id);
           const unlockedDecryptionKeys = new Map(state.unlockedDecryptionKeys);
           unlockedDecryptionKeys.delete(id);
-          return { unlockedKeys, unlockedDecryptionKeys };
+          const unlockedLegacyDecryptionKeys = new Map(state.unlockedLegacyDecryptionKeys);
+          unlockedLegacyDecryptionKeys.delete(id);
+          return { unlockedKeys, unlockedDecryptionKeys, unlockedLegacyDecryptionKeys };
         });
       },
 
       lockAllKeys: () => {
         clearRememberedUnlocks();
-        set({ unlockedKeys: new Map(), unlockedDecryptionKeys: new Map() });
+        set({ unlockedKeys: new Map(), unlockedDecryptionKeys: new Map(), unlockedLegacyDecryptionKeys: new Map() });
       },
 
       getKeyRecordForIdentity: (identityId) => {
@@ -476,6 +496,7 @@ export const useSmimeStore = create<SmimeStore>()(
           publicCerts: [],
           unlockedKeys: new Map(),
           unlockedDecryptionKeys: new Map(),
+          unlockedLegacyDecryptionKeys: new Map(),
           identityKeyBindings: {},
           defaultSignIdentity: {},
           defaultEncrypt: false,