Sendmail to duplicate messages

[Olivier Nicole]

Hi,

What rule, in what file should I set-up to have my mail sendmail mail
server duplicate all messages to another mail server?

I am in the proces sof setting-up a new mail server; in the meantime I
want all messages arriving to my current mail server under sendmail to
be duplicated to the new mail server (under postfix, for all it
matters); so the mailboxes are keept in sync.

How can I do that with sendmail?

TIA,

Olivier