If I understand your question, you want to act as a backup MX for xdomain.com.
The DNS records for xdomain.com should contain at least two MX records similar to this:
xdomain.com. 8172 IN MX 5 smtp.xdomain.com.
xdomain.com. 8172 IN MX 10 smtp.isp.com.
Notice that the higher-numbered record points to the backup server. This will cause any mail destined for xdomain.com to go to smtp.isp.com in the event that smtp.xdomain.com is down.
There's not much that needs to be done on the ISP's postfix server to act as a backup for xdomain.com.
relay_domains = xdomain.com
relay_recipient_maps = hash:/etc/postfix/relay_recipients
The first line makes tells postfix to accept mail for xdomain.com, but to hold on to it. Postfix will do an MX lookup on xdomain.com and deliver the mail to smtp.xdomain.com when it becomes available.
The second line defines all the valid recipients for xdomain.com (which avoids accepting mail for nonexist usernames, i.e. avoids spam).
/etc/postfix/relay_recipients might look something like this:
Hash tables require a value, so we put "dummy". In this case, the value is not used.
Finally, we build the .db file and reload postfix: