Unable to send emails

I am located in NJ and 3 days ago I realized I was unable to send emails through my Apple Mail program. Receiving is still working. I believe the problem occurred after resetting my modem. I checked to make sure my port settings were correct and they seem to be, but still the problem persists. I'm working on an old system, Mac OSX 10.6.8, and Apple Mail version 4.6. Please let me know if anyone out there has suggestions or if more information is needed i would be happy to provide it to figure this out, thank you!