[Exim] Sending mail from anywhere

Hi,

we are using an external webserver to recieve mails which we download
via pop3. We configured exim on that server to forward the mails in the
right mailboxes. We send mail via an internal mailserver, so we do not
use exim to send mail in our company.

The big question is: How can I send mail from home, using user/password
authentication?
I want to send mail via exim from home (without our internal mailserver
in the company, but via exim on the external server), but the IP address
at home is not static, so I would like to have exim checked the
user/password information which is stored in the password file.

I'm rather new to this, so I apologize if a) this is trivial, or b) this
is not possible. Reading the documentation since last week, I feel more
confused every day :-)

Best regards,
Chrsitian von Appen