I must be doing something wrong. I am new to thunderbird, migrated from Eudora since I can't send mail with Eudora anymore.

I tried to configure Thunderbird as much as possible like Eudora, but ... it doesn't receive nor send mail. On the surface it looks fine, it seems thunderbird logs into the server, it seems it sends mail and checks for new mail, only I don't receive anything, nor send anything even though it looks it does. So what am I doing (or is going) wrong? (I can still receive mail with Eudora and today I checked and received 54 mails, none with Thunderbird)