Frizz

I'm having such a nightmare with my email. I was using Eudora and all was fine until last Thursday when out of the blue I couldn't get my emails. I kept looking at the preferences trying to see what was up, but it all looked OK and I could still send emails. I wrote to the server, but they didn't offer any useful suggestions. So as a last resort I reverted back to Mail, successfully imported everything from Eudora and it worked for the first day and a half - now I can't send any emails using my normal email address. So I can only send using Eudora and only download using Mail, which is kinda crazy. I have triple checked and both programs have the same settings, I don't understand what is going on.