How to make one virtual address in sendmail

[a at zeos.net]

I use sendmail 8.13.6 and
I want all mail from my local host to "my-external-mailbox at external.host.name"
is going to "my-local-mailbox at localhost"

How to? Can I use any standard feature?

I can add

my-external-mailbox at external.host.name	my-local-mailbox at localhost

in virtusertable, but it will nor work without

external.host.name

in local-host-names, or VIRTUSER_DOMAIN(`external.host.name'), 
or any other ruler putting 
"external.host.name" in class {w} or something like that.
But I cannot make the last because of
my sendmail will try to deliver locally all mail from local host to 
"external.host.name". The sender of the mail will receive "User unknown" 
immediately (except for the "my-local-mailbox at localhost") and nothing more.

Thank you in advance.

Elisej Babenko