There are two different ways to check your messages in ResBook. First one is on the left hand side of your screen, inside the Calendar/Home tag.
Click on 
> click on 
> click on ‘Messages’. There you could check a single message or all of them by selecting ‘View all messages’.
Another choice is by clicking 
on the top menu. This is the same place that you get to by going through the ‘View all messages’ option from the way listed above.
Once you come to here, you can check the Message Subject, who it is from, sent date, status or choose to delete the message.