Troubleshooting sendmail smart_host relay session

[T o n g <mlist4suntong@yahoo.com>]

Hi, 

Somehow my sendmail SMART_HOST stops working now,

- It was working fine, and I didn't change any sendmail configurations.
- Test sending mails through my ISP's SMTP sever using Thunderbird proved 
to be working fine. 

The, /var/log/mail.log says, DSN: User unknown:

  $ grep 14778 /var/log/mail.log
  Mar 16 21:12:56 helios sendmail[14778]: gethostbyaddr(192.168.0.104) failed: 1
  Mar 16 21:12:56 helios sendmail[14778]: n2H1CuXA014778: from=acct@isp.net, size=15, class=0, nrcpts=1, msgid=<200903170112.n2H1CuXA014778@helios.selfip.org>, relay=root@localhost
  Mar 16 21:12:56 helios sendmail[14778]: n2H1CuXA014778: to=user@external.com, ctladdr=acct@isp.net (0/0), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30015, relay=mailrouter1.isp.net. [199.166.6.105], dsn=5.1.1, stat=User unknown
  Mar 16 21:12:56 helios sendmail[14778]: n2H1CuXA014778: n2H1CuXB014778: DSN: User unknown
  Mar 16 21:12:57 helios sendmail[14778]: n2H1CuXB014778: to=acct@isp.net, delay=00:00:01, xdelay=00:00:01, mailer=relay, pri=30000, relay=mailrouter1.isp.net. [199.166.6.105], dsn=2.0.0, stat=Sent (OK id=1LjNrT-0005d1-L4)

The actual session that sendmail relayed messages through smart_host:

  % (echo subject: test; echo) | sendmail -Am -v -f acct@isp.net user@external.com
  user@external.com... Connecting to mailrouter1.isp.net. via relay...
  220 mailrouter4.execulink.net ESMTP Exim 4.43 Mon, 16 Mar 2009 21:12:55 -0400
  >>> EHLO helios.selfip.org
  250-mailrouter4.execulink.net Hello node-13261.pppoe.isp.net [216.75.189.214]
  250-SIZE 20971520
  250-PIPELINING
  250-AUTH PLAIN LOGIN
  250 HELP
  >>> MAIL From:<acct@isp.net> SIZE=15 AUTH=acct@isp.net
  250 OK
  >>> RCPT To:<user@external.com>
  >>> DATA
  550 relay not permitted
  503 valid RCPT command must precede DATA
  >>> RSET
  250 Reset OK
  >>> RSET
  250 Reset OK
  acct@isp.net... Using cached ESMTP connection to mailrouter1.isp.net. via relay...
  >>> MAIL From:<>
  250 OK
  >>> RCPT To:<acct@isp.net>
  >>> DATA
  250 Accepted
  354 Enter message, ending with "." on a line by itself
  >>> .
  250 OK id=1LjNrT-0005d1-L4
  acct@isp.net... Sent (OK id=1LjNrT-0005d1-L4)
  Closing connection to mailrouter1.isp.net.
  >>> QUIT
  221 mailrouter4.execulink.net closing connection

Anyone has any idea what could be wrong? (I'm using Debian Lenny)

Thanks a lot!