Directory Alias/Virtual

I've given it a go and I'm doing somethign wrong.. I swear it..

got a few virtual name domains going and I want a generic /webmail for
each domain one:

123.com/webmail
xyz.com/webmail

to point to squirrelmail or a directory somewhere else on the system to
service html content

so I put in the domani config:

alias /directorytosquirrelmail/squirrelmail /webmail

am I being completely thick? I've had a look at some documentation but
it doesn't provide simple examples for me to understand :(

Daz