diff --git a/lib/api/mailboxes.js b/lib/api/mailboxes.js
index 0c3216a7..323ebe7e 100644
--- a/lib/api/mailboxes.js
+++ b/lib/api/mailboxes.js
@@ -34,6 +34,7 @@ module.exports = (db, server, mailboxHandler) => {
      * @apiParam {String} user Users unique ID
      * @apiParam {Boolean} [specialUse=false] Should the response include only folders with specialUse flag set.
      * @apiParam {Boolean} [counters=false] Should the response include counters (total + unseen). Counters come with some overhead.
+     * @apiParam {Boolean} [sizes=false] Should the response include mailbox size in bytes. Size numbers come with a lot of overhead as an aggregated query is ran.
      *
      * @apiSuccess {Boolean} success Indicates successful response
      * @apiSuccess {Object[]} results List of user mailboxes
@@ -107,6 +108,10 @@ module.exports = (db, server, mailboxHandler) => {
                     .truthy(['Y', 'true', 'yes', 'on', '1', 1])
                     .falsy(['N', 'false', 'no', 'off', '0', 0, ''])
                     .default(false),
+                sizes: Joi.boolean()
+                    .truthy(['Y', 'true', 'yes', 'on', '1', 1])
+                    .falsy(['N', 'false', 'no', 'off', '0', 0, ''])
+                    .default(false),
                 sess: Joi.string().max(255),
                 ip: Joi.string().ip({
                     version: ['ipv4', 'ipv6'],
@@ -138,6 +143,9 @@ module.exports = (db, server, mailboxHandler) => {
 
             let user = new ObjectID(result.value.user);
             let counters = result.value.counters;
+            let sizes = result.value.sizes;
+
+            let sizeValues = false;
 
             let userData;
             try {
@@ -166,6 +174,37 @@ module.exports = (db, server, mailboxHandler) => {
                 return next();
             }
 
+            if (sizes) {
+                try {
+                    sizeValues = await db.database
+                        .collection('messages')
+                        .aggregate([
+                            {
+                                $match: {
+                                    user
+                                }
+                            },
+                            {
+                                $project: {
+                                    mailbox: '$mailbox',
+                                    size: '$size'
+                                }
+                            },
+                            {
+                                $group: {
+                                    _id: '$mailbox',
+                                    mailboxSize: {
+                                        $sum: '$size'
+                                    }
+                                }
+                            }
+                        ])
+                        .toArray();
+                } catch (err) {
+                    // ignore
+                }
+            }
+
             let mailboxes;
             try {
                 mailboxes = await db.database
@@ -224,6 +263,15 @@ module.exports = (db, server, mailboxHandler) => {
                     subscribed: mailboxData.subscribed
                 };
 
+                if (sizeValues) {
+                    for (let sizeValue of sizeValues) {
+                        if (mailboxData._id.equals(sizeValue._id)) {
+                            response.size = sizeValue.mailboxSize;
+                            break;
+                        }
+                    }
+                }
+
                 if (!counters) {
                     responses.push(response);
                     continue;