You didn’t specify, but I’m assuming this is a Windows network, all on TCP/IP. Otherwise, the answer will be off base.
Do you know if the RAS system is allowing NetBIOS broadcasts to get through?
Also, are the remote users authenticating to a domain controller? Or is this a workgroup?
Since your users can get through email-wise, then the issue is not network connectivity per se, but smells like an MS issue.
Just for jollies, have a test remote user try “net view \serverIP” from the command line to see what happens.