Can't send mail on leopard

[oldmanmax]

I can receive mail from my provider and from .mac, but can not send. Keeps telling me to try another server. Did mail doctor and it says I can connect to my providers server and to mac's server and smtp server, but I can not connect to my provider's smtp, and the smtp settings are correct per my provider, they can not find any reason I can not connect. I have tried making a new account thinking it would make things better, no good.
Anyone have any suggestions?
Using G5 ppc, Leopard 10.5.4