installation

I am testing with D6.8. Download and install Mail API then install IMAPWU API then apply http://drupal.org/node/279980 if it is not yet included.
Now install Webmail Plus. You may need a webmail_plus.info if not supplied. I created the following file. (This page would not let me attach it.)

name = Webmail Plus
description = Webmail Plus is a full-featured email client for Drupal.
package = Webmail Plus
core = 6.x
version = "6.x-1.x-dev"
dependencies[] = mail_api
dependencies[] = imapwu_api

Webmail Plus includes the following so I guess you need them.

require_once "PEAR.php";
require_once "Mail/RFC822.php";

My site found PEAR but not Mail/RFC822.php. I downloaded Mail-1.1.14.tgz from the PEAR site, expanded the file, then copied the Mail subdirectory into the webmail_plus directory.

In webmail_plus.module, I changed
module_exist( to module_exists(
theme_add_style( to drupal_add_css(

Webmail Plus started installing, ran into a Javascript error and another include error.

A start on documentation based on 6.x-1.4: http://drupal.org/node/369604