MAILHOST /QMAILHOST environment variable

Hi all..

Sme 6.0.1 in server and gateway setup..

domain name is something like 'myname.local'

smarthost is my isp smtp server..

Every administrative mail (from cron daemon ecc) is sent to an external mail address..

That's what I get:

--------------------------------------------------------------
From: MAILER-DAEMON@myname.local
To: root@myname.local
Subject: failure notice

Hi. This is the qmail-send program at myname.local.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
<external_address>:
Connected to SMTP_IP_SERVER but sender was rejected.
Remote host said: 553 <root@myname.local> unable to verify address
.........
--------------------------------------------------------------

so I've googled a bit and I've found MAILHOST env variable here

http://cr.yp.to/qmail/faq/appearance.html#user-masquerading

tested with
export MAILHOST='another_domain.tld'

mail -s 'test' external_address < test.txt

from root console.. works perfectly

Well, that's the question..

Where can I add MAILHOST env variable so that make it persistent?

TIA

Stefano