You can actually set iis to send outgoing smtp traffic on any port - could use 5252 - anything above 1024.
The convention for smtp is port 25, which comcast has started blocking for pc's on their network.
Port 2525 is an alternate port that is sometimes hardcoded into 3rd party programs, proxy servers, and firewalls as an alternative and is just easy to remember.
Comcast lets you use their smtp servers though, so you probably can just specify them unless you're using something like dns2go to have your own domain on a dynamic ip.
Hotmail, AOL, and Yahoo are all tightening up on their spam rules though. Expect to see more rejected email if your smtp server can't be mapped to a static ip associated with your domain name in dns spf records.
Edited by dpyers