Plesk windows webmail 503 error

It happened that webmail from plesk windows just opened a page with error 503 , service unavailable. In our case it was horde webmail with plesk 8.6  IIS7 on windows 2008. Restarting whole IIS did not help.

Unfortunately I did not find too many docs related to it so find out from Parallels that webmail is not running from “webmail” application pool but on plesk default application pool that can be found in plesk -> server-> IIS Application Pool . If this is not running so webmail will not work and you have to restart this application pool to start webmail.