sendmail is rejecting email from 1 domain b/c "spf check failed" but the sender is valid

Hi, I recently set up sendmail to serve as a relay server for inbound email

external email ---> sendmail server ----> corporate server

the sendmail server seems to work fine, but one domain (xyz.com) gets its emails bounced with the error of "550 Message rejected because SPF check failed".  the person sending mail from xyz.com **can** deliver mail successfully to gmail and hotmail accounts, so the problem appears to be on my end.

If I look at the spf records online for xyz.com (e.g. using an spf tester like this-> http://www.kitterman.com/spf/validate.html) it says that the the IP address that my sender is sending from is valid.

Are there any simple ways to disable spf checking on my relay server?  Or other ways around this?
Thanks!!
pdaneseAsked:
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pdaneseAuthor Commented:
sorry for the lack of points, but I'm all out!
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jar3817Commented:
You didn't mention what software is doing the SPF checking. By default, sendmail doesn't do it, you usually need a milter like milter-spiff or mimedefang. Once we know that we can figure out how to bypass it.

The is probably due to the admin of xyz.com not setting SPF up correctly or not updating it when addresses changed. They can deliver to gmail and such because I don't think gmail will reject mail when it fails spf checking. Your system is a little stricter.

Sadly this is pretty common. I have a few vendors I work with who just can't seem to get their SPF records right and as a result always land in my spam folder, even after I repeated tell them how to fix it.
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pdaneseAuthor Commented:
Hi, so here's the weird part (may help shed some light on the problem).

1. if I look at the headers of the emails sent from xyz.com I can find the IP address from which they originated.  BUT the IP address is listed as "valid" if I do a SPF chek

2. I used ubuntu 8 and installed sendmail on it (apt-get install sendmail).  I did a few modifications to enable the relay, but I never installed anything remotely like SPF software.  Also, searching the server turns up no file with the term spf in it.

So...I'm thinking that I don't really have SPF checking enabled, but for whatever reason it provides that error upon bouncing certain emails.

One other thing!  If I look at the mail.log file I also see messages of "DSN: service unavailable" near the messages that are getting bounced back.

my google searches have turned up little as to what that means, though.

Thanks again!
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jar3817Commented:
Can you paste the (all) entires in the /var/log/maillog for one of these bounced messages? If you didn't specifically setup SPF checking, I'm sure it's not checking them.
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pdaneseAuthor Commented:
Hi, here's an excerpt (I had to obfuscate the actual addresses).

Oct 22 08:51:32 ubuntu sm-mta[17430]: m9MCpQkK017430: from=<xxxxxx.x.xxxx@xyz.com>, size=13384, class=0, nrcpts=1, msgid=<C081D7F029F55E4BA2AFBE026A2088210DE5988F@xxxxxxm02.xxxx.xyz.com>, proto=ESMTP, daemon=MTA, relay=xxxxxx.xyz.com [123.456.789.01]
Oct 22 08:51:32 ubuntu sm-mta[17438]: m9MCpLAX017432: m9MCpWAX017438: DSN: Service unavailable
Oct 22 08:51:32 ubuntu sm-mta[17436]: m9MCpLU3017431: m9MCpWU3017436: DSN: Service unavailable
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pdaneseAuthor Commented:
sorry for the delay in points
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