[exim] multiple MTA tries

Hi all,
At office, I must use one MTA (smtp without auth)
At home, I must use another MTA (smtp without auth)
My backup (GPRS) internet connection must use one MTA (smtp without auth)

Each one rejects (5xx) me if I "smtp without auth" in the wrong place.

I would like to setup an intelligent Exim MTA on my laptop
that would try (smtp without auth) each MTA and use the first
accepting.

I will then setup my MUA to use "localhost" as MTA and then the
localhost Exim will manage.

How to?
Thank you.

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