diff --git a/README.md b/README.md
index dea4f288..b914fe28 100644
--- a/README.md
+++ b/README.md
@@ -193,7 +193,7 @@ Shard the following collections by these keys:
 sh.enableSharding('wildduck');
 sh.shardCollection('wildduck.messages', { mailbox: 1, uid: 1 });
 sh.shardCollection('wildduck.threads', { user: 'hashed' });
-sh.shardCollection('wildduck.attachments.files', { 'metadata.h': 'hashed' });
+sh.shardCollection('wildduck.attachments.files', { _id: 'hashed' });
 sh.shardCollection('wildduck.attachments.chunks', { files_id: 'hashed' });
 ```
 
diff --git a/api.js b/api.js
index cf6a5bad..ac4b211f 100644
--- a/api.js
+++ b/api.js
@@ -104,8 +104,9 @@ module.exports = done => {
     messageHandler = new MessageHandler({
         database: db.database,
         users: db.users,
+        redis: db.redis,
         gridfs: db.gridfs,
-        redis: db.redis
+        attachments: config.attachments
     });
 
     userHandler = new UserHandler({
diff --git a/config/default.toml b/config/default.toml
index bb46d115..92bac061 100644
--- a/config/default.toml
+++ b/config/default.toml
@@ -52,6 +52,10 @@ maxForwards=2000
     # used to push outbound emails to the sending queue
     #sender="zone-mta"
 
+[attachments]
+    type="gridstore"
+    bucket="attachments"
+
 [log]
     level="silly"
     # log to syslog if true
diff --git a/imap.js b/imap.js
index 49286418..490071aa 100644
--- a/imap.js
+++ b/imap.js
@@ -84,60 +84,6 @@ let mailboxHandler;
 let gcTimeout;
 let gcLock;
 
-function deleteOrphanedAttachments(callback) {
-    // NB! scattered query
-    let cursor = db.gridfs.collection('attachments.files').find({
-        'metadata.c': 0,
-        'metadata.m': 0
-    });
-
-    let deleted = 0;
-    let processNext = () => {
-        cursor.next((err, attachment) => {
-            if (err) {
-                return callback(err);
-            }
-            if (!attachment) {
-                return cursor.close(() => {
-                    // delete all attachments that do not have any active links to message objects
-                    callback(null, deleted);
-                });
-            }
-
-            if (!attachment || (attachment.metadata && attachment.metadata.c)) {
-                // skip
-                return processNext();
-            }
-
-            // delete file entry first
-            db.gridfs.collection('attachments.files').deleteOne({
-                _id: attachment._id,
-                // make sure that we do not delete a message that is already re-used
-                'metadata.c': 0,
-                'metadata.m': 0
-            }, (err, result) => {
-                if (err || !result.deletedCount) {
-                    return processNext();
-                }
-
-                // delete data chunks
-                db.gridfs.collection('attachments.chunks').deleteMany({
-                    files_id: attachment._id
-                }, err => {
-                    if (err) {
-                        // ignore as we don't really care if we have orphans or not
-                    }
-
-                    deleted++;
-                    processNext();
-                });
-            });
-        });
-    };
-
-    processNext();
-}
-
 function clearExpiredMessages() {
     clearTimeout(gcTimeout);
     let startTime = Date.now();
@@ -181,7 +127,7 @@ function clearExpiredMessages() {
 
         if (config.imap.disableRetention) {
             // delete all attachments that do not have any active links to message objects
-            return deleteOrphanedAttachments(() => done(null, true));
+            return messageHandler.attachmentStorage.deleteOrphaned(() => done(null, true));
         }
 
         // find and delete all messages that are expired
@@ -208,7 +154,7 @@ function clearExpiredMessages() {
         let clear = () =>
             cursor.close(() => {
                 // delete all attachments that do not have any active links to message objects
-                deleteOrphanedAttachments(() => {
+                messageHandler.attachmentStorage.deleteOrphaned(() => {
                     if (deleted) {
                         server.logger.debug(
                             {
@@ -297,8 +243,9 @@ module.exports = done => {
 
         messageHandler = new MessageHandler({
             database: db.database,
+            redis: db.redis,
             gridfs: db.gridfs,
-            redis: db.redis
+            attachments: config.attachments
         });
 
         userHandler = new UserHandler({
diff --git a/indexes.yaml b/indexes.yaml
index 4cb87df8..908b5cf2 100644
--- a/indexes.yaml
+++ b/indexes.yaml
@@ -253,6 +253,12 @@ indexes:
 # attachments.files collection should be sharded by _id (hash)
 # attachments.chunks collection should be sharded by files_id (hash)
 
+- collection: attachments.files
+  type: gridfs # index applies to gridfs database
+  index:
+    name: attachment_id_hashed
+    key:
+      _id: hashed
 - collection: attachments.files
   type: gridfs # index applies to gridfs database
   index:
diff --git a/lib/attachment-storage.js b/lib/attachment-storage.js
index 0817c72b..123c9964 100644
--- a/lib/attachment-storage.js
+++ b/lib/attachment-storage.js
@@ -1,8 +1,7 @@
 'use strict';
 
-const ObjectID = require('mongodb').ObjectID;
+const GridstoreStorage = require('./attachments/gridstore-storage.js');
 const crypto = require('crypto');
-const GridFSBucket = require('mongodb').GridFSBucket;
 let cryptoAsync;
 try {
     cryptoAsync = require('@ronomon/crypto-async'); // eslint-disable-line global-require
@@ -12,30 +11,20 @@ try {
 
 class AttachmentStorage {
     constructor(options) {
-        this.bucketName = options.bucket || 'attachments';
-        this.gridfs = options.gridfs;
-        this.gridstore = new GridFSBucket(this.gridfs, {
-            bucketName: this.bucketName
-        });
+        this.options = options || {};
+
+        let type = (options.options && options.options.type) || 'gridstore';
+
+        switch (type) {
+            case 'gridstore':
+            default:
+                this.storage = new GridstoreStorage(this.options);
+                break;
+        }
     }
 
     get(attachmentId, callback) {
-        this.gridfs.collection('attachments.files').findOne({
-            _id: attachmentId
-        }, (err, attachmentData) => {
-            if (err) {
-                return callback(err);
-            }
-            if (!attachmentData) {
-                return callback(new Error('This attachment does not exist'));
-            }
-
-            return callback(null, {
-                contentType: attachmentData.contentType,
-                transferEncoding: attachmentData.metadata.transferEncoding,
-                metadata: attachmentData.metadata
-            });
-        });
+        return this.storage.get(attachmentId, callback);
     }
 
     create(attachment, callback) {
@@ -43,68 +32,12 @@ class AttachmentStorage {
             if (err) {
                 return callback(err);
             }
-
-            this.gridfs.collection(this.bucketName + '.files').findOneAndUpdate({
-                'metadata.h': hash
-            }, {
-                $inc: {
-                    'metadata.c': 1,
-                    'metadata.m': attachment.magic
-                }
-            }, {
-                returnOriginal: false
-            }, (err, result) => {
-                if (err) {
-                    return callback(err);
-                }
-
-                if (result && result.value) {
-                    return callback(null, result.value._id);
-                }
-
-                let returned = false;
-
-                let id = new ObjectID();
-                let metadata = {
-                    h: hash,
-                    m: attachment.magic,
-                    c: 1,
-                    transferEncoding: attachment.transferEncoding
-                };
-                Object.keys(attachment.metadata || {}).forEach(key => {
-                    if (!(key in attachment.metadata)) {
-                        metadata[key] = attachment.metadata[key];
-                    }
-                });
-
-                let store = this.gridstore.openUploadStreamWithId(id, null, {
-                    contentType: attachment.contentType,
-                    metadata
-                });
-
-                store.once('error', err => {
-                    if (returned) {
-                        return;
-                    }
-                    returned = true;
-                    callback(err);
-                });
-
-                store.once('finish', () => {
-                    if (returned) {
-                        return;
-                    }
-                    returned = true;
-                    return callback(null, id);
-                });
-
-                store.end(attachment.body);
-            });
+            return this.storage.create(attachment, hash, callback);
         });
     }
 
     createReadStream(id) {
-        return this.gridstore.openDownloadStream(id);
+        return this.storage.createReadStream(id);
     }
 
     deleteMany(ids, magic, callback) {
@@ -120,57 +53,15 @@ class AttachmentStorage {
     }
 
     updateMany(ids, count, magic, callback) {
-        // update attachments
-        this.gridfs.collection(this.bucketName + '.files').updateMany(
-            {
-                _id: {
-                    $in: ids
-                }
-            },
-            {
-                $inc: {
-                    'metadata.c': count,
-                    'metadata.m': magic
-                }
-            },
-            {
-                multi: true,
-                w: 1
-            },
-            callback
-        );
+        this.storage.update(ids, count, magic, callback);
     }
 
     delete(id, magic, callback) {
-        this.gridfs.collection(this.bucketName + '.files').findOneAndUpdate({
-            _id: id
-        }, {
-            $inc: {
-                'metadata.c': -1,
-                'metadata.m': -magic
-            }
-        }, {
-            returnOriginal: false
-        }, (err, result) => {
-            if (err) {
-                return callback(err);
-            }
+        this.storage.delete(id, magic, callback);
+    }
 
-            if (!result || !result.value) {
-                return callback(null, false);
-            }
-
-            if (result.value.metadata.c === 0 && result.value.metadata.m === 0) {
-                return this.gridstore.delete(id, err => {
-                    if (err) {
-                        return callback(err);
-                    }
-                    callback(null, 1);
-                });
-            }
-
-            return callback(null, 0);
-        });
+    deleteOrphaned(callback) {
+        this.storage.deleteOrphaned(callback);
     }
 
     calculateHash(input, callback) {
diff --git a/lib/attachments/gridstore-storage.js b/lib/attachments/gridstore-storage.js
new file mode 100644
index 00000000..03aeb778
--- /dev/null
+++ b/lib/attachments/gridstore-storage.js
@@ -0,0 +1,217 @@
+'use strict';
+
+const ObjectID = require('mongodb').ObjectID;
+const GridFSBucket = require('mongodb').GridFSBucket;
+
+class GridstoreStorage {
+    constructor(options) {
+        this.bucketName = (options.options && options.options.bucket) || 'attachments';
+        this.gridfs = options.gridfs;
+        this.gridstore = new GridFSBucket(this.gridfs, {
+            bucketName: this.bucketName
+        });
+    }
+
+    get(attachmentId, callback) {
+        this.gridfs.collection(this.bucketName + '.files').findOne({
+            _id: attachmentId
+        }, (err, attachmentData) => {
+            if (err) {
+                return callback(err);
+            }
+            if (!attachmentData) {
+                return callback(new Error('This attachment does not exist'));
+            }
+
+            return callback(null, {
+                contentType: attachmentData.contentType,
+                transferEncoding: attachmentData.metadata.transferEncoding,
+                length: attachmentData.length,
+                count: attachmentData.metadata.c,
+                hash: attachmentData.metadata.h,
+                metadata: attachmentData.metadata
+            });
+        });
+    }
+
+    create(attachment, hash, callback) {
+        this.gridfs.collection(this.bucketName + '.files').findOneAndUpdate({
+            'metadata.h': hash
+        }, {
+            $inc: {
+                'metadata.c': 1,
+                'metadata.m': attachment.magic
+            }
+        }, {
+            returnOriginal: false
+        }, (err, result) => {
+            if (err) {
+                return callback(err);
+            }
+
+            if (result && result.value) {
+                return callback(null, result.value._id);
+            }
+
+            let returned = false;
+
+            let id = new ObjectID();
+            let metadata = {
+                h: hash,
+                m: attachment.magic,
+                c: 1,
+                transferEncoding: attachment.transferEncoding
+            };
+            Object.keys(attachment.metadata || {}).forEach(key => {
+                if (!(key in attachment.metadata)) {
+                    metadata[key] = attachment.metadata[key];
+                }
+            });
+
+            let store = this.gridstore.openUploadStreamWithId(id, null, {
+                contentType: attachment.contentType,
+                metadata
+            });
+
+            store.once('error', err => {
+                if (returned) {
+                    return;
+                }
+                returned = true;
+                callback(err);
+            });
+
+            store.once('finish', () => {
+                if (returned) {
+                    return;
+                }
+                returned = true;
+                return callback(null, id);
+            });
+
+            store.end(attachment.body);
+        });
+    }
+
+    createReadStream(id) {
+        return this.gridstore.openDownloadStream(id);
+    }
+
+    delete(id, magic, callback) {
+        this.gridfs.collection(this.bucketName + '.files').findOneAndUpdate({
+            _id: id
+        }, {
+            $inc: {
+                'metadata.c': -1,
+                'metadata.m': -magic
+            }
+        }, {
+            returnOriginal: false
+        }, (err, result) => {
+            if (err) {
+                return callback(err);
+            }
+
+            if (!result || !result.value) {
+                return callback(null, false);
+            }
+
+            /*
+            // disabled as it is preferred that attachments are not deleted immediately but
+            // after a while by a cleanup process. This gives the opportunity to reuse the
+            // attachment
+
+            if (result.value.metadata.c === 0 && result.value.metadata.m === 0) {
+                return this.gridstore.delete(id, err => {
+                    if (err) {
+                        return callback(err);
+                    }
+                    callback(null, 1);
+                });
+            }
+            */
+
+            return callback(null, true);
+        });
+    }
+
+    update(ids, count, magic, callback) {
+        // update attachments
+        this.gridfs.collection(this.bucketName + '.files').updateMany(
+            {
+                _id: Array.isArray(ids)
+                    ? {
+                        $in: ids
+                    }
+                    : ids
+            },
+            {
+                $inc: {
+                    'metadata.c': count,
+                    'metadata.m': magic
+                }
+            },
+            {
+                multi: true,
+                w: 1
+            },
+            callback
+        );
+    }
+
+    deleteOrphaned(callback) {
+        // NB! scattered query
+        let cursor = this.gridfs.collection(this.bucketName + '.files').find({
+            'metadata.c': 0,
+            'metadata.m': 0
+        });
+
+        let deleted = 0;
+        let processNext = () => {
+            cursor.next((err, attachment) => {
+                if (err) {
+                    return callback(err);
+                }
+                if (!attachment) {
+                    return cursor.close(() => {
+                        // delete all attachments that do not have any active links to message objects
+                        callback(null, deleted);
+                    });
+                }
+
+                if (!attachment || (attachment.metadata && attachment.metadata.c)) {
+                    // skip
+                    return processNext();
+                }
+
+                // delete file entry first
+                this.gridfs.collection('attachments.files').deleteOne({
+                    _id: attachment._id,
+                    // make sure that we do not delete a message that is already re-used
+                    'metadata.c': 0,
+                    'metadata.m': 0
+                }, (err, result) => {
+                    if (err || !result.deletedCount) {
+                        return processNext();
+                    }
+
+                    // delete data chunks
+                    this.gridfs.collection('attachments.chunks').deleteMany({
+                        files_id: attachment._id
+                    }, err => {
+                        if (err) {
+                            // ignore as we don't really care if we have orphans or not
+                        }
+
+                        deleted++;
+                        processNext();
+                    });
+                });
+            });
+        };
+
+        processNext();
+    }
+}
+
+module.exports = GridstoreStorage;
diff --git a/lib/message-handler.js b/lib/message-handler.js
index 84df5f41..f55d747f 100644
--- a/lib/message-handler.js
+++ b/lib/message-handler.js
@@ -26,7 +26,8 @@ class MessageHandler {
         this.attachmentStorage =
             options.attachmentStorage ||
             new AttachmentStorage({
-                gridfs: options.gridfs || options.database
+                gridfs: options.gridfs || options.database,
+                options: options.attachments
             });
 
         this.indexer = new Indexer({
diff --git a/lmtp.js b/lmtp.js
index 5f7e9ecb..733d3bc1 100644
--- a/lmtp.js
+++ b/lmtp.js
@@ -434,9 +434,10 @@ module.exports = done => {
 
     messageHandler = new MessageHandler({
         database: db.database,
-        gridfs: db.gridfs,
         users: db.users,
-        redis: db.redis
+        redis: db.redis,
+        gridfs: db.gridfs,
+        attachments: config.attachments
     });
 
     let started = false;
diff --git a/pop3.js b/pop3.js
index f7d760d5..2881c6b5 100644
--- a/pop3.js
+++ b/pop3.js
@@ -309,8 +309,9 @@ module.exports = done => {
 
     messageHandler = new MessageHandler({
         database: db.database,
+        redis: db.redis,
         gridfs: db.gridfs,
-        redis: db.redis
+        attachments: config.attachments
     });
 
     userHandler = new UserHandler({