I have an SBS 2008 server that is using ATT DSL with a dynamic ip address. (please no lectures :p) I can receive mail all day long, but I cannot send mail at all. I assume this is because ATT has port 25 blocked for their residential customers. I currently have a dyndns set up to assist with mail delivery. How can I get my outbound mail working, please know that I'm fairly new to exchange and may not understand everything posted. Thanks a bunch!
The fact that you're using Exchange is unlikely to be relevant here.
Traditionally AT&T has allowed you to opt-out of the port 25 blocking, though the documented page for requesting an automatic opt-out seems to have gone missing. For residential customers, you may need to log in to your account where you'd manage features in order to get to this.
Failing that, if you have a website hosted elsewhere, you can probably set up your Send Connector to use your web host as a Smart Host (see http://technet.microsoft.com/en-us/library/aa998814%28EXCHG.80%29.aspx for instructions on setting up authentication, etc.), but check with them first. You might also be able to set Exchange to prefer using its own connections on 465/587, but you may not get messages accepted at all remote sites.
See also this for configuring Smart Host authentication: http://technet.microsoft.com/en-us/library/bb629654%28EXCHG.80%29.aspx
Have you configured a Send Connector to send outbound mail? There isn't one by default.
If you HAVE, then yes, AT&T is probably blocking port 25 traffic. You are likely going to have to relay through their mailserver, and that's something you'll have to get the settings for from them.