ok, i try that (using apache 1.3.xx) :
in /etc/apache/conf.d/vhosts.conf
Code:
NameVirtualHost *:80
<VirtualHost *:80>
ServerAdmin dz@mydom.com
DocumentRoot /var/www/myweb
ServerName myweb.mydom.com
ErrorLog /var/log/apache/myweb-error.log
CustomLog /var/log/apache/myweb-access.log common
</VirtualHost>
<VirtualHost *:80>
ServerName zimbra.mydom.com
ServerAdmin dz@mydom.com
ProxyPass / http://zimbra.mydom.com:81/
ProxyPassReverse / http://zimbra.mydom.com:81/
ErrorLog /var/log/apache/zimbra-error.log
CustomLog /var/log/apache/zimbra-access.log common
</VirtualHost> I tried myweb.mydom.com, everythings fine 
I tried to connect to zimbra.mydom.com :
Code:
An unknown application error has occurred.
Please correct any errors and retry.
If the problem persists, please contact your System
Aministrator. (service.FAILURE)
ok, no luck .. trying zimbra.mydom.com ... same error.
the apache proxy seems to work but zimbra appears to hate this 
or maybe zimbra just fall (some service seems to be down), i dont know.
going to check logs brb