Not sure how Wireshark would help, unless you manage to capture the flow at the exact moment the problem appears.

Besides

> One user reported that the client hanged while typing an email on server1

The only thing we know here is that the client hanged. That's not much to incriminate the network. In true client/server fashion, no network traffic happens while typing. It may be more something related to the client configuration or the workstation.

> his colleague said that db´s on server 2 was not reachable, so notes hanged.

Server 2 being not reachable is one thing, but in itself should not be enough to make Notes hang. Notes was designed around intermittent connectivity. If a server is unreachable, it tells you so, and moves on (after a couple of seconds).

Before calling in the Wireshark cavalry, you could maybe try and refine connectivity diagnosis by activating some built-in tools : see here.

I would also check the local console.log files in Data/IBM_TECHNICAL_SUPPORT

On the client side too, Preferences/Notes Ports/Trace could give you some hints as to the route taken to resolve the server address and give you pointers to look for possible occasional conflicts.

Checking the relevant Server document in the N&AB, and specialy the field "Fully qualified Internet host name", could give you some pointers too.

Also, check the server's log.nsf, specially around the time the server was last started. If something's askew as to network connectivity, some grunt should be registered here. Same thing around the time of reported incident. See to (temporaryly) increase logging verbosity.

Finally, see the Domino Diagnostic Probe.