Is there a way for a user to have a smtp account of mail.yourdomain.com instead of using the ispconfig server domain? Can the username be changed from the default webXX_username naming convention? Which files/directories do you need to back up? And how easy is it to restore these files/dirs if you need to perform a recovery?
you can use sender_dependent_relayhost_maps feature. http://www.postfix.org/postconf.5.html#sender_dependent_relayhost_maps Hope this helps. Dipesh