diff --git a/app/internal_packages/composer/lib/composer-view.jsx b/app/internal_packages/composer/lib/composer-view.jsx
index f8678d336..58681780b 100644
--- a/app/internal_packages/composer/lib/composer-view.jsx
+++ b/app/internal_packages/composer/lib/composer-view.jsx
@@ -533,7 +533,7 @@ export default class ComposerView extends React.Component {
             return;
           }
 
-          match.contentId = Utils.generateTempId();
+          match.contentId = Utils.generateContentId();
           session.changes.add({
             files: [].concat(draft.files),
           });
diff --git a/app/internal_packages/composer/lib/inline-image-upload-container.jsx b/app/internal_packages/composer/lib/inline-image-upload-container.jsx
index d4aa18677..9a3169f0b 100644
--- a/app/internal_packages/composer/lib/inline-image-upload-container.jsx
+++ b/app/internal_packages/composer/lib/inline-image-upload-container.jsx
@@ -147,12 +147,12 @@ export default class InlineImageUploadContainer extends Component {
       return <span />;
     }
     if (isPreview) {
-      return <img src={`cid:${file.id}`} alt={file.name} />;
+      return <img src={`cid:${file.contentId}`} alt={file.name} />;
     }
 
     return (
       <div
-        data-src={`cid:${file.id}`}
+        data-src={`cid:${file.contentId}`}
         className="inline-image-upload-container"
         onDoubleClick={this._onGoEdit}
       >
diff --git a/app/src/flux/models/utils.coffee b/app/src/flux/models/utils.coffee
index 57f95e1c4..b2c861055 100644
--- a/app/src/flux/models/utils.coffee
+++ b/app/src/flux/models/utils.coffee
@@ -155,6 +155,11 @@ Utils =
       Math.floor((1 + Math.random()) * 0x10000).toString(16).substring(1)
     'local-' + s4() + s4() + '-' + s4()
 
+  generateContentId: ->
+    s4 = ->
+      Math.floor((1 + Math.random()) * 0x10000).toString(16).substring(1)
+    'mcid-' + s4() + s4() + '-' + s4()
+
   isTempId: (id) ->
     return false unless id and _.isString(id)
     id[0..5] is 'local-'
diff --git a/app/src/flux/stores/attachment-store.es6 b/app/src/flux/stores/attachment-store.es6
index 61c5266e1..d65cbbb79 100644
--- a/app/src/flux/stores/attachment-store.es6
+++ b/app/src/flux/stores/attachment-store.es6
@@ -436,7 +436,7 @@ class AttachmentStore extends MailspringStore {
         size: stats.size,
         contentType: null,
         messageId: null,
-        contentId: inline ? Utils.generateTempId() : null,
+        contentId: inline ? Utils.generateContentId() : null,
       });
 
       await mkdirpAsync(path.dirname(this.pathForFile(file)));
diff --git a/app/src/flux/stores/draft-helpers.es6 b/app/src/flux/stores/draft-helpers.es6
index 6f7ecdb93..a0932ce5f 100644
--- a/app/src/flux/stores/draft-helpers.es6
+++ b/app/src/flux/stores/draft-helpers.es6
@@ -59,7 +59,7 @@ class DraftHelpers {
 
   async pruneRemovedInlineFiles(draft) {
     draft.files = draft.files.filter(f => {
-      return !(f.contentId && !draft.body.includes(`cid:${f.id}`));
+      return !(f.contentId && !draft.body.includes(`cid:${f.contentId}`));
     });
 
     return draft;
diff --git a/mailsync b/mailsync
index f5625c3c8..d4b6bdc8b 160000
--- a/mailsync
+++ b/mailsync
@@ -1 +1 @@
-Subproject commit f5625c3c8d102856a2c4b838e9d40f751b1df216
+Subproject commit d4b6bdc8b9ef6fec0023d59c94e9f71de72f99ef