First, let me welcome you to Tek-Tips.
I had to re-read your post several times and I think I may understand your question. Please correct me if I am wrong, but I interpret your statements to mean that you are running an email server and you allow others (clients) to send mail via this server. Your concern is that if one of them uses your server to SPAM that your IP address will get black listed and this will cause problems for your OTHER, non spamming, clients.
There are a couple of problems you face.
First, the situation as described is true. The fact is that by allowing these clients to send mail through your server, you are accepting some responsibility for their actions. If one of them does start spamming, YOU as the mail server operator and YOUR server will bear the consequence. The first thing I would advise is that you reconsider the wisdom of this action.
If you decide to continue with this approach, you should think about using stringent outbound content filtering and rejecting any mail from a client that appears to be SPAM.
Second, from a technical standpoint, it sounds like you are trying to "spoof" the IP address - "dynamically change the IP based upon the originating domain." This is technically complex and difficult because the transactions involved in the email require bidirectional communication. This communication uses the IP address to properly route. I can't think of anyway to dynamically "assign" your server a different (public) IP on a per transaction basis.
Third, even if you do manage to change the originating IP on a per email or a per client basis, you will undoubtedly find yourself getting black listed. It is an unfortunate reality that SPAM is a real problem and a lot of spam originates with zombie computers that are not true, officially designated, email servers. Consequently, to run an email server you need to have solid credentials behind it, like forward and reverse DNS, domain keys, SPF records, STATIC IP addresses, etc. Trying to run an email server that changes its IP address would be highly suspicious.
