If you are using a secure SMTP server, the default port (port 25) is disabled by many ISP's in the states in order to cut down on spam. what is the specific error message that you receive? My ISP just cut off port 25 one day without any warning. I assumed it was my e-mail provider until i checked further. I ended up just calling the ISP and they turned it back on for me...after warning me they would be monitoring it for SPAM and such.
We could probably answer your question much better if there was a better description of your setup and the error you receive, however
