Can't send mail, can receive

Hope this is the correct forum for a mail question. I am running a Powerbook with OS 10.5.8 with mail 3.6. For no obvious reason, mail quit sending messages. It receives but does not send. I have reinstalled mail from the installation disc but it didn't resolve the problem. I doubt it was a factor but when the send function quit I was on a very slow dial-up connection. During this time when I was lucky enough to get connected, the connection would drop after a minute or two or it might last for several minutes. This happened repeatedly while I was trying to download received mail and a program.

Thank you for any suggestions you may have.

Ted Stampfer