How can I change qMail's ip address?
I have 4 IP addresses on my server. 126.96.36.199 is blacklisted and i want using 188.8.131.52 for outgoing mails.
qmail uses the first IP address it finds to send mail from. If you want to change this behaviour, you have to patch it and recompile. If this scares, worries or concerns you then maybe qmail is not for you.
There are patches available that do this:
Also, with regards to Plesk, someone suggested here to:
Also see this question which seems highly relevant to your situation.
If you switch to Postfix,
I do hope you are also dealing with the reason your IP address was blacklisted.
You're Doing It Wrong
In fact, you're Doing It Wrong in two ways:
Stop using QMail. It's horribly antiquated and hasn't been updated in 14 YEARS.
That might not be the answer you're looking for, but it's the correct answer for anyone still using it.