I can't give you the official answer, but I can tell you what worked for me.
I edited the http.conf file in /etc/httpd/conf
I added the following line:
Alias \webmail \home\e-smith\files\primary\html\horde\imp
I added it here:
Alias \webmail \home\e-smith\files\primary\html\horde\imp
#====================
# End of aliases
#====================
Then did a - service httpd restart - on the console
The upside is it works. The downside is that if something ever triggers a
/sbin/e-smith/signal event/console-save, then your settings will be lost and you have to re-input them, and restart httpd. For me I tested this and did lose the settings in the httpd.conf file, but the /webmail still worked. I tested it on several machines. I am not using a proxy server at all.
I looked at editing the templates, specifically 80Aliases00, in /etc/e-smith/templates/etc/hpttd/conf/httpd.conf, but it had some code in it. Being a newbie, I didn't want to change or mess it up, and I don't know how to add my own template, or use the template-begin file.
Hope this helps.