Clues to send mail on LAN through Postfix?

[Vinicius]

Hello,

I'm trying to send a message on LAN through Postfix, but Postfix can't 
resolve the host name. Any clues, please?

myhost.mydomain resolves to 10.20.30.40 by /etc/hosts.


TIA, Vinicius.

Sending a message:
"
# mail -v -s 'Testing sending message' root
Testing sending message.
Cc:
root... Connecting to [127.0.0.1] via relay...
220 myhost ESMTP Postfix (2.3.3)
 >>> EHLO myhost.mydomain
250-myhost
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
 >>> MAIL From:<root at myhost.mydomain> SIZE=74
250 2.1.0 Ok
 >>> RCPT To:<root at myhost.mydomain>
 >>> DATA
250 2.1.5 Ok
354 End data with <CR><LF>.<CR><LF>
 >>> .
250 2.0.0 Ok: queued as 4627A33DD83
root... Sent (Ok: queued as 4627A33DD83)
Closing connection to [127.0.0.1]
 >>> QUIT
221 2.0.0 Bye
"

/var/log/maillog :
"
Nov 14 10:45:58 myhost postfix/postfix-script: starting the Postfix mail 
system
Nov 14 10:45:58 myhost postfix/master[24964]: daemon started -- version 
2.3.3, configuration /etc/postfix
Nov 14 10:46:41 myhost sendmail[24980]: kAECkfsm024980: from=root, 
size=74, class=0, nrcpts=1, msgid=<200611141246.kAECkfsm024
980 at myhost.mydomain>, relay=root at localhost
Nov 14 10:46:41 myhost postfix/smtpd[24983]: connect from 
localhost.localdomain[127.0.0.1]
Nov 14 10:46:41 myhost postfix/smtpd[24983]: 4627A33DD83: 
client=localhost.localdomain[127.0.0.1]
Nov 14 10:46:41 myhost postfix/cleanup[24986]: 4627A33DD83: 
message-id=<200611141246.kAECkfsm024980 at myhost.mydomain>
Nov 14 10:46:41 myhost postfix/qmgr[24966]: 4627A33DD83: 
from=<root at myhost.mydomain>, size=538, nrcpt=1 (queue active)
Nov 14 10:46:41 myhost sendmail[24980]: kAECkfsm024980: to=root, 
ctladdr=root (0/0), delay=00:00:00, xdelay=00:00:00, mailer=r
elay, pri=30074, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent 
(Ok: queued as 4627A33DD83)
Nov 14 10:46:41 myhost postfix/smtpd[24983]: disconnect from 
localhost.localdomain[127.0.0.1]
Nov 14 10:46:51 myhost postfix/smtp[24987]: 4627A33DD83: 
to=<root at myhost.mydomain>, relay=none, delay=11, delays=0.05/0.01/11/0,
  dsn=4.4.3, status=deferred (Host or domain name not found. Name 
service error for name=myhost.mydomain type=A: Host not found,
  try again)
"