Issue IP address & Helo for outgoing Mails are domain based?

[Mailinghoster]

Hi, today I tried for an customer the following scenario:

We have on our Server an domain like mylink.com with an dedicated IP therefore we have created an SMTP Account. In Plesk -> Outgoing mail mode is "Send from domain IP addresses and use domain names in SMTP greeting". If I send a test mail for example with Webmail or Outlook and use info@mylink.com as FROM: address to send a mail all work good and the dedicated IP & right Helo are taken.

But now comes the problem ;-)

Our customer want to use his own FROM: - address for example info@customeremail.com in email and send it over our SMTP. No Problem till now. I changed it in our Mailsystem to force using his FROM: - address and our SMTP.

But if I send now a testmail over the smtp, the mail will be send over the default Server IP & Helo and not over the dedicated IP. If I changed it back to original SMTP FROM - address all is working again.

So I think plesk / postfix don't recognize, what route should be used when I change the FROM - address...

So my Questions are:
Must we allow anywhere that it is possible to use an other sending FROM as original SMTP From ?
Must we create an other routing for example in /etc/postfix/master.cf ?