replacing port in outgoing packets to any host

[ck]

Hello, participants!

In constant effort to prevent trojans to send spam following question 
came to my mind.

Is there any way to replace port number for all outgoing packets?

Long version:

I want to block outgoing port 25 completely for network behind NAT 
router and allow port 8025 for example. But it means that router will 
have to replace outgoing port 8025 with port 25. After intensive 
googling it looks like my idea is... well... not popular. So, I just 
wonder if this is possible at all? Something like this:

rdr any to any port 8025 -> any port 25

PS Yes, I know that I can redirect port to open-relay on known static IP.

Thanks!