How to setup mail to conform to expected behaviour?

[mxc]

Hi all,

How does one setup the mail server in ispconfig so that people using a mail client can enter the following to get their mail:

username:user@customerdomian.
password:secret password
incoming mail server: mail.customerdomain


I can do the last by creating a c-name record to www.<customerdomain> and then entering that as the mx record for the dns entry. How do I do the rest? Currently I can use the email address as the userid for the web mail login but when I try and use it for a mail client such as thunderbird or fetchmail I get authentication errors. Otherwise I have to enter username like web2_username.

This is especially important as I want to migrate users to the new ispconfig server. I want to cause as little diruption to them as possible. Getting them to change their email account setting will be a nightmare. Trying to explain that you use different login details for web based mail as opposed to the email client is a problem. Not to mention people forgetting their web2_username login details

thanks