It's not all that uncommon for mail to go to multiple addresses, but if neither fred nor frank knew jim was getting copies of their mail then they should challenge jim. However, whether 'jim' may claim it is a mistake, they certainly will have known "since it was set up" and if it was a mistake could have acted to rectify it.
It does seem odd that jim knows what comes in for frank (and/or fred - without sending individual mail messages, it isn't clear from the example whether only fred's mail, or only frank's mail, is copied to jim
Assuming they are 'equal' partners, can see no reason why jim should feel it necessary or even acceptable for jim to get copies of mail to others, without the reverse. On the other hand, I'm always cautious about sending anything to a relative using their 'work' mail address.
(It's a bit daft, in my view, for any employee to not have their own mail account very separate from work, in case they are making applications for alternative jobs or have some problem at work and need a secure means to contact union or legal representatives. Also, all mail in a work environment might be logged and reviewed, so completely daft for anything unrelated to work, IMO. Sorry, just an aside on benefit of having multiple mail addresses.)