Hi, I'm having this Sendmail configuration problem and stuck. I've been doing googling/reading posts and none of the solutions matched my problem. I'm doing some testing, and have been getting this 550 5.1.1 ....User unknown error when I send the email to my company's email address. When I send it to an external email address, such as yahoo, the message delivered successfully. Of course, I'm doing the configuration behind a firewall. I'm using a relay too, and I did declare the relay host in 'SMART_HOST' in the sendmail.mc file and have also recompiled it to produce the sendmail.cf. I also restarted the sendmail service. ---------------------------------------------------------------------------------------------------------------------------------------------------- telnet ana.boeing.com 25 Trying 134.51.151.114... Connected to ana.boeing.com (134.51.151.114). Escape character is '^]'. 220 ana.boeing.com ESMTP Sendmail 8.12.11.20060308/8.12.11; Sat, 1 May 2010 16:39:06 -0700 helo ana.boeing.com 250 ana.boeing.com Hello ana.boeing.com [134.51.151.114], pleased to meet you mail from:root at ana.boeing.com 250 2.1.0 root at ana.boeing.com... Sender ok rcpt to:mary.y.wang at boeing.com 550 5.1.1 mary.y.wang at boeing.com... User unknown Any ideas? I'm running on Sendmail 8.12.11 Mary