Some more info. Below is a more complete telnet session from backup mail server (mx2.greenspot.fi) to primary mail server (mail.greenspot.fi). It only proves that sending mail works fine from command-line. But my sendmail setup cannot do the same. I wrote that MAILER-DAEMON can get mail through. I am not actually sure about that. (One warning message did come through, but maybe it was because something I changed in the config - not sure anymore.) Here's the successful telnet session: [root at mx2 ~]# telnet mail.greenspot.fi 25 Trying 83.143.217.182... Connected to mail.greenspot.fi (83.143.217.182). Escape character is '^]'. 220 mail.greenspot.fi ESMTP Sendmail; Tue, 15 May 2012 23:17:59 +0300 HELO mx2.greenspot.fi 250 mail.greenspot.fi Hello mx2.greenspot.fi [83.143.217.179], pleased to meet you MAIL FROM: me1 at domain.com 250 2.1.0 me1 at domain.com... Sender ok RCPT TO: me2 at domain.com 250 2.1.5 me2 at domain.com... Recipient ok DATA 354 Enter mail, end with "." on a line by itself Testing testing . 250 2.0.0 q4FKHxf7012785 Message accepted for delivery QUIT 221 2.0.0 mail.greenspot.fi closing connection Connection closed by foreign host. - Jussi