The IMAP server will need support for sieve. Looks like you would want to check with ghandi.net or whoever your email server is to see if they support it.
Did you install Roundcube? If so, then in Roundcube you would need to configure the managesieve plugin. After verifying if the server supports it, and you may need some port numbers and such.
If you did not install Roundcube then go to the Roundcube About menu it should show a list of installed plugins, if it is not there you probably need to check with your website host to see what is possible.