Can I use a variable as a class name?

I have a further question related to the iRule in this thread. I forgot to mention that what I want to also do it to rewrite the destination port to a specific port, actually port 25.

 

 

However, I don't see any way to achieve this without having a pool attached. Since this is outbound traffic, and I'm using a widlcard forwarindg virtual server to attach the iRule to, then I don't see how this can be achieved. Any thoughts?