[exim] sending mail from outside

Hello,

I'm very new to exim and as I don't have time (yet) to read the manual
I'm trying to quickly find a solution to the following problem :

We have an exim4 server running with SMTP to directly send and receive
emails. It works very well from any boxes on the local network.

When I am outside the network, that is at home, I still want to be able
to read my office emails. It works. But if I want to reply or send any
mail from home using my office email account, then it doesn't work.

Is there any simple procedure to set up exim4 for this purpose ?

I couldn't find any answer to this question, may be trying the wrong
keywords to search for ?! ...

Cheers and thanks a lot for your help

--
Samuel Thoraval
LIBROPHYT, Bioinformatique
Centre de Cadarache
Bâtiment 185, DEVM
13108 St Paul-Lez-Durance
France
Tél: +33 442 574 799
Fax: +33 442 574 439
e-mail : samuel.thoraval@???