sendmail: real domain name required for sender address

Is there something new or broken in sendmail (8.14.7-1.fc19)? This is the error message I got in the rejected mail corresponding to the logwatch report. I cannot anymore send mail locally with sendmail... what a pain...

Does anybody have a clue about it? It always worked "out of the box" before...

Anyway, I switched to postfix and everything is fine.