I need to configure an e-mail server(Postfix/Linux/CentOs) in a VPS, I need to get my e-mail delivered at 95%.
My main question is: Should I use the VPS where I have the website or should I use a new VPS with other IP only dedicated to mail delivery?
You don't post any details on how saturated your current site is. If it's hardly used, there's no problem using it to send email.
If it's busy, use another VM with it's own resources.
If you're using it on a hosted service like Amazon, there's no guarantee you won't have occasional slowdowns from neighboring VM's anyway. There's not much guarantee of resource choking or random slowdowns.
And I'm not sure what you mean by "delivered at 95%"...successful deliveries? You can get higher than that if you know for sure you're sending to correct email addresses. Are you having errors due to issues with configuration or messages seen as spam or...? You can only control so many factors here...
