SPF check failed

I recently got a new customer that can't send us email.  I see a log that says SPF check failed, and that's all it says.  If they can't send to me, I'm thinking it would be their SPF record causing the problem.  I have never dealt with SPF records, so I'm a newbie here.  We are having no issues sending emails, and this one domain seems to be the only domain having issues sending us emails.  I see a txt record in my DNS that says the following:  

v=spf1 mx -all

its been there for years, I've never quite understood it so I never got rid of it.  Any ideas on what this means?
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Dave BaldwinFixer of ProblemsCommented:
Here's the 'official' web site http://www.openspf.org/  and here's the Wikipedia article that may make more sense: http://en.wikipedia.org/wiki/Sender_Policy_Framework

If he has an SPF text record in his DNS, he may be sending from a domain that is not listed there which would cause his messages to be blocked.

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Dave BaldwinFixer of ProblemsCommented:
denver218Author Commented:
Thanks.  What does the following mean though?

v=spf1 mx -all
Sudeep SharmaTechnical DesignerCommented:
v=spf1 is the version of spf used. Here it is version 1.
mx ==> Specifies all A records for each host's MX record. Here you are matching the IP connected to your mail server with the MX records (A records) gathered from the DNS.

~all ==> Softfail. The address failed the test, but the result is not definitive. So even if this failed the test email would still be accepted.

Details here:

denver218Author Commented:
It ended up being SPF record on the remote side.
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