situation-specific relaying... can this be done?

So I've got a weird situation happening here. The college I'm attending has done some weird things with mail, and now I need to work around it. specificly, most of the usual ports for SMTP are restricted to on-campus usage only, which leaves 25 as the only viable option - which my ISP takes care of in short order. So here's what I'm hoping to accomplish. I'd like to set up a relay from the CPanel server that hosts my personal email so that email I sent from my college email, through that server, is relayed to the servers the college uses for mail reception. I only want this to effect outbound email, if possible - so if someone from the college opts to email my personal address, it doesn't trigger interesting and inventive ways to loop itself to death. I've seen threads mentioning doing things with static routes, and I'm giving them some serious thought, but that's my only real concern - if someone from the college emails, I don't want it tripping the static route, and causing itself an infinite loop. Is there a way I can work around this? Thanks for the help either way. PS: would be nice, but not absolutely required, if I can also convince it to support SMTP Auth - just in case the college doesn't much care for what I'm doing.