the e-mail address is already configurable in the web interface - it is the address of the root@pam user, which you can edit when editing that user (see /usr/bin/pvemailforward.pl for how the forwarding is actually implemented, it's rather simple).
what about pre-filling the mail address field with some invalid example address in the installer like we do for the FQDN? that way, you would have an @ already there if you cannot enter one yourself, but it would still be mandatory to change it to a real value.. you'd just have to pay attention to not accidentally delete the @
what about pre-filling the mail address field with some invalid example address in the installer like we do for the FQDN? that way, you would have an @ already there if you cannot enter one yourself, but it would still be mandatory to change it to a real value.. you'd just have to pay attention to not accidentally delete the @