An easy fix would be to use your ISP's SMTP server in your email client.
In Outlook (express) there is an advanced option. In here you need to add the username/password of your ISP.
When you set the POP3 to your own server, your email client will receive all the email from it.
It will use the SMTP from your ISP to send the email.
The email address shown will be the one you use in your email client (does not need to be the one from your ISP)
Small note. Hotmail might/will still see your send email as spam (as the SPF record is not correct for your ISPconfig domain), and will endup in the bulk folder.
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