I think that this cannot be done, because sendmail is not just a program that is run standalone, but a daemon. On a server, this needs to control port 25, so there can only be one sendmail (or other smtp service). If you allow a subscription to access the daemon, the daemon could be controlled by that subscription e.g. stopped, restarted, reconfigured etc., which in turn could badly affect other subscriptions (they'd become dependent on the mercy of the one subscription owner who can access sendmail). It is not possible to have a second, alternative instance of sendmail in the chrooted environment, because this would conflict with the port 25 requirement. The only thing you could maybe do is to have another smtp service run on a different port, but what sense would this make? However, of course you can use the daemon's service from the chrooted environment (meaning you can use port 25 to send mail through it).