Howdy,
Here is what I ended up doing.
DNS
I defined a webmail.<our domain> CNAME record in DNS to resolve to our current smtp server IP
I also defined an email.<our domain> CNAME record in DNS to resolve to the same address.
Apache
I created rewrite rule in the /etc/apache2/httpd.conf file to catch the incoming URL and
rewrite the URL to the correct path.
Here are the httpd.conf file lines that I added for the virtual host.
<VirtualHost <our internal non-routable IP> <our external routable IP>>
ServerName email.<our domain>
ServerAlias webmail.<our domain>t
RewriteEngine on
RewriteCond %{HTTP_HOST} email\.<our domain> [OR]
RewriteCond %{HTTP_HOST} webmail\.<our doamin>
RewriteRule .*
http://smtp.<our doamin>/cgi-bin/webmail\.cg
i [R=permanent]
UserDir disabled
DocumentRoot /tmp
ServerAdmin admin@<our domain>
ErrorLog /<internal path>/email-error.log
</VirtualHost>