Open the config file unbuffered to help with concurrency.

exec/imapservice.js

@@ -848,7 +848,7 @@ var unauthenticated_command_handlers = {
 					return;
 				}
 				cfgfile=new File(format(system.data_dir+"user/%04d.imap", user.number));
-				if (!cfgfile.open(cfgfile.exists ? 'r+':'w+', true)) {
+				if (!cfgfile.open(cfgfile.exists ? 'r+':'w+', true, 0)) {
 					tagged(tag, "NO", "Can't open imap state file");
 					return;
 				}