Do you mean an alternative to Horde IMP webmail? Horde is what is (only) shipped as webmail with Plesk. However, there are at least 3 possibilities:
1) Install another webmail client via plesk Application Vault (Web applications), for example ilohamail Webmail program http://ilohamail.org/ or http://www.uebimiau.org/
2) create an alias record (CNAME) in the domain DNS settings redirecting to your webmail page IP
3) create a subdomain webmail.domain.com and put an index.html file with all the content
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