Make backup MX server configurable in GUI
when you set you backup server in /etc/postfix/main.cf after ISPConfig update it's gone Imagine - you set ISPC server for customer, make MX backup server in DNS. Then customers update ISPC, relay setting for backup MX is gone and all email sent via backup MX will never arrive.
default postfix config by ISPC is: relay_domains = mysql:/etc/postfix/mysql-virtual_relaydomains.cf
i use: relay_domains = backup.mx.tld mysql:/etc/postfix/mysql-virtual_relaydomains.cf
but imo more elegant way will be put backup mx serer into SQL somewhere