Think you’d only want to do that for networks you completely trust. Any network you add to mynetworks will be able to use your mailserver to relay/send mail. If they send spam etc it’ll completely ruin your reputation and you’ll not be able to send email anywhere.
I don’t trust the domain thats having the issue enough to want to allow them to relay email out of my server, but do want to be able to receive email destined only to my users from them.