+=item XAPIAN_FLUSH_THRESHOLD
+
+The number of documents to update before committing changes to
+disk. This environment is handled directly by Xapian, refer to
+Xapian API documentation for more details.
+
+Default: our indexing code flushes every megabyte of mail seen
+to keep memory usage low. Setting this environment variable to
+any positive value will switch to a document count-based
+threshold in Xapian.
+