sending email outside from sendmail

[homebase]

I just set up a mail server using using sendmail.
I dont have a registered domain name but i am using a
dyndns account and have set it up to forward mails.
I am able to recieve mails from outside ( from hotmail, yahoo etc.)
but i can't send mail outside ( to hotmail, etc).
any one has an idea as to what i am doing wrong?
any tutorials on such a senerio?

[fnmblot]

Your ISP is probably blocking your port 25. Try changing the port to 2525 or something like that.

[homebase]

any idea as to how to do that?
How do i do it on my server?
and how do i configure the dyndns account to use 2525

[fnmblot]

afaik you have to pay for the port forwarding from dyndns. you change the port in the config file. Search the forum or google and exactly how to do that.