To Redirect to the Horde Webmail Client: 

RewriteEngine on
RewriteCond %{HTTP_HOST} ^webmail.purdonlaw.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.webmail.purdonlaw.com$
RewriteRule ^(.*)$ http://purdonlaw.com:2095/horde/$1 [R=301,L]

To Redirect to the Squirrel Mail Client: 

RewriteEngine on
RewriteCond %{HTTP_HOST} ^webmail.purdonlaw.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.webmail.purdonlaw.com$
RewriteRule ^(.*)$ http://purdonlaw.com:2095/3rdparty/squirrelmail/index.php$1 [R=301,L]