diff --git a/src/backuptask.js b/src/backuptask.js
index c512c9ab1..6a33a7923 100644
--- a/src/backuptask.js
+++ b/src/backuptask.js
@@ -60,7 +60,7 @@ async function checkPreconditions(backupConfig, dataLayout) {
     let used = 0;
     for (const localPath of dataLayout.localPaths()) {
         debug(`checkPreconditions: getting disk usage of ${localPath}`);
-        const result = safe.child_process.execSync(`du -Dsb --exclude='*.lock' "${localPath}"`, { encoding: 'utf8' });
+        const result = safe.child_process.execSync(`du -Dsb --exclude='*.lock' --exclude='dovecot.list.index.log.*' "${localPath}"`, { encoding: 'utf8' });
         if (!result) throw new BoxError(BoxError.FS_ERROR, `du error: ${safe.error.message}`);
         used += parseInt(result, 10);
     }