RCP over HTTP and Internal outlook

When users are internal and the Client is setup for RPC externally, what
stops the client from connecting to the front end (keeps prompting for
credentials when internal) so I can have the client just connect to it's
mailbox on the backend like a normal MAPI when Internal.

the msstd: will have the client goto the front end ect. when they are
internal,, how do I get back integrated mapi logon (locally) and still
support front end RPC when they are remote,

Thanks for any info