You could do this with a simple RewriteCond/RewriteRule in the domain's vhost.conf file (not overwritten by Plesk), or if you prefer to make changes in httpd.conf/httpd.include, you could change /webmail to /correo. But remember these files can and will be overwritten by Plesk during upgrades and such.
These are just 2 easy methods, there are other ways as well (ServerAlias, etc). Personally I would do the Rewrite method, if for no other reason that I don't want to have to remember to re-modify the httpd files later....