We use a client v7.0.3 with a mailbox in v7 too. The server is a Domino V8.0.1.The user wait more than 30 seconds when he try to open is mailbox after a long idle time (more than 15 minutes).
No problem when the connection is etablished with the server, until a new long dle time.
The first question would be…is this happening to just one user or multiple users?
If this is only one user then I would look at that user’s mailfile and client setup (connection doc, location doc).
If this is happening to multiple users then I would run a trace (file > preferences > user preferences > ports) from one of the clients to the server to see the path it is using to connect to the server.