[exim] From line in e-mails vs. vacation

Hi there,

I have a debian 7.6 server (two.hoes.li) set up with myself as first user
with username "stef". (This reflects in the exim 4.80 configuration as:
errors_to = stef)

The issue I have is the following: I have an user set up called
webmaster-alt and a virtual domain on that server that excepts e-mails to
webmaster.alt@<VIRTUAL DOMAIN> that gets delivered to webmaster-alt.

For webmaster-alt I have a .forward file containing the following:

\webmaster-alt, "|/usr/bin/vacation webmaster-alt -r 0 -a webmaster.alt"

This doesn't work as expected, since all the e-mails in
/var/mail/webmaster-alt start with:

>From stef@???...

Accordingly stef@??? get all the vacation auto-responses and not
the original sender.

Why does exim start every delivered e-mail with "From stef@???..."?

Regards
Stef