ServerAlias and a RedirectRule?
Untested and quickly written before I got my first coffee:

<host xyz>
    ServerName client.domain.com
    ServerAlias  webmail.client.domain.com
    RewriteEngine on
    RewriteCond %{HOST_NAME} ^webmail.client.domain.com$
    RewriteRule ^/$ http://webmail.client.domain.com/cgi-bin/webmail [
+L,R]
</host>
[download]

I hope that helps to get you started.

regards,
tomte

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Edit:Added the [L,R] part
Edit 2:AFAIK this works with http/1.1 Clients only, as http/1.0-clients don't send a Host-Header

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