Security with transport map settings?

Hi Everybody,

We have a smtp gateway which deliver mails for the domain: "@paris.info" and
use postfix as MTA. Postfix is configured to forward mails to only one of
our internal mail server.

That is to say, in /etc/postfix/transport:

paris.info smtp:10.0.5.1

But in order to have a good quality of mail service, and if the 10.0.5.1
mail server is down, no mail will be deliver.

My question is: how can I tell postfix, if my primary mail server is down,
to deliver mail to another mail server, like 10.0.5.2? do I use transport
maps settings?

Thank you for yours answers.

Baud