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cyberchrisrockFlag for United States of America

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550 Host unknown

I have built a Linux Fedora 12 box as a MTA for the company to facilitate a new Exchange
server that is on the inside network. The MTA is siting on the DMZ and can ping the Exchange
server and visa versa. When I try to send mail to the test account which is setup on the exchange
server, I get the follwoing error:

----- The following addresses had permanent fatal errors ----- <>
    (reason: 550 Host unknown)

I have used every tool and test that I can think of or find, and I cannot fathom why this error
exist or continue to.The company has a Bonded T1 service from Covad. I called and spoke to them
about this but they had no real answer except to buy more services and had over the in house mail
operation. I included their smtp information in the "Smart Host" section of the Sendamil config file.
I have tried to be as comprehensive as possible so that I can get an answer as quick as possible,
without fraustrating you guys and myself. I was also able to telnet to the MTA via
"telnet 25"
Would someone please give me a hand with this?

Here are the results of all the tests:

nslookup results

Non-authoritative answer:
Address:  w.x.y.z

More nslookup results
> set type=mx
Address:  10.x.x.x

Non-authoritative answer:       MX preference = 0, mail exchanger =       nameserver =       nameserver =  internet address = w.x.y.z
--DNS Records SPF Text value--

v=spf1 a mx ptr mx:w.x.y.z ip4:w.x.y.z-all

--DNS A Record--

mail      w.x.y.z

--DNS MX Record--

0      @


--output from SMTP Diag via

220 ESMTP Sendmail 8.14.3/8.14.3; Sat, 13 Feb 2010 22:34:54 -0600

 Not an open relay.
 0 seconds - Good on Connection time
 0.094 seconds - Good on Transaction time
 OK - resolves to
 Warning - Reverse DNS does not match SMTP Banner

Session Transcript:
250 Hello [] (may be forged), pleased to meet you [31 ms]
250 2.1.0 <>... Sender ok [16 ms]
550 5.7.1 <>... Relaying denied. IP name possibly forged [] [31 ms]
221 2.0.0 closing connection [16 ms]

--Port Scan via

       25      smtp      Success      31 ms

These ports were closed:

       21      ftp      Timeout      0 ms
       22      ssh      Timeout      0 ms
       23      telnet      Timeout      0 ms
       53      dns      Timeout      0 ms
       80      http      Timeout      0 ms
       110      pop3      Timeout      0 ms
       143      imap      Timeout      0 ms
       139      netbios      Timeout      0 ms
       389      ldap      Timeout      0 ms
       443      https      Timeout      0 ms
       587      msa-outlook      Timeout      0 ms
       1433      sql server      Timeout      0 ms
       3306      my sql      Timeout      0 ms
       3389      remote desktop      Timeout      0 ms
       8080      webcache      Timeout      0 ms

--Result from

Result: Ok
MX record about exists.
Connection succeeded to SMTP.
220 ESMTP Sendmail 8.14.3/8.14.3; Sun, 14 Feb 2010 21:14:11 -0600
250 Hello [], pleased to meet you
=250 2.1.0 <>... Sender ok
> RCPT TO: <>
=250 2.1.5 <>... Recipient ok

--Results from DNS

Searching for A record at G.ROOT-SERVERS.NET. [] ...took 31 ms
Searching for A record at [] ...took 120 ms
Searching for NS32.DOMAINCONTROL.COM. A record at H.ROOT-SERVERS.NET. [] ...took 95 ms
Searching for NS32.DOMAINCONTROL.COM. A record at [] ...took 165 ms
Searching for A record at NS32.DOMAINCONTROL.COM. [] ...took 92 ms


---Partial SendMail config---

      dnl # Uncomment and edit the following line if your outgoing mail needs to      
Other      dnl # be sent out through an external mail server:      
Other      dnl #      
Other      dnl define(`SMART_HOST', `')dnl      
Other      dnl #      
Define      define(`confDEF_USER_ID', ``8:12'')dnl      
Other      dnl define(`confAUTO_REBUILD')dnl      
Define      define(`confTO_CONNECT', `1m')dnl      
Define      define(`confTRY_NULL_MX_LIST', `True')dnl      
Define      define(`confDONT_PROBE_INTERFACES', `True')dnl      
Define      define(`PROCMAIL_MAILER_PATH', `/usr/bin/procmail')dnl      
Define      define(`ALIAS_FILE', `/etc/aliases')dnl      
Define      define(`STATUS_FILE', `/var/log/mail/statistics')dnl      
Define      define(`UUCP_MAILER_MAX', `2000000')dnl      
Define      define(`confUSERDB_SPEC', `/etc/mail/userdb.db')dnl      
Define      define(`confPRIVACY_FLAGS', `authwarnings,novrfy,noexpn,restrictqrun')dnl      
Define      define(`confAUTH_OPTIONS', `A')dnl

NOTE THE "SMART_HOST" section in the sendmail config above.

Thanks in Advance

Avatar of eSouth

Have you verified that you can send mail to that user from another user that's on the same Exchange server?  A 550 error could also be associated with the user as with the host.
Avatar of jar3817

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Avatar of cyberchrisrock


This morning a got a pleasant surprise. The in box of the test account ( not only had mail, from all the test that I sent, it also had mail from 2 days past. The only explanation I have is that the DNS information was not totaly propagated "world wide".  I did not change any configurations sice I posted last night. Anyway the problem that I have left is listed below, after sending mail to an external address:

#550 4.4.7 QUEUE.Expired; message expired in unreachable destination queue. Reason: A matching connector cannot be found to route the external recipient ##"

As it plainly states, the "Connector" settings in Exchange, is not quite right, I will visit those and post the outcome as soon as it becomes available.
It turns out as I said in the previous post it started work when the DNS propagated. I setup the connector settings in exchange .. The problem is solved.