Proper formatting for DNS SPF record

We have been getting a fair amount of NDR's from emails that were clearly not send by our users, yet have our users email addresses as the return to value.  I suspect it could be due to improper formatting of the SPF entry in our DNS zone.  Below is the line.  Is it correct?

Note:  we have three servers that send mail.  Our primary exchange server "exchangeserver" and two application servers that send emails to our clients "mailapplicationserver1 and 2"


Our SPF record:
;;ourdomain.com. 3600 IN TXT "v=spf1 mx ptr a:exchangeserver.ourdomain.com a:mailapplicationserver1.ourdomain.com a:mailapplicationserver2.ourdomain.com ~all"
mchad65Asked:
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Chris DentPowerShell DeveloperCommented:
It's called Backscatter, which doesn't help much except if you're Googling :)

SPF is not totally reliable. It can help reduce the cause of back scatter, but you are utterly reliant on other mail system administrators paying attention to it. Adoption is far better than it was, but it's far from being 100%.

There are a couple of ways around this, both are a bit focused on you having at least Exchange 2007.

You can remove the ability for anonymous senders to use your addresses (when mail is received by your system, not globally):

http://exchangepedia.com/blog/2008/09/how-to-prevent-annoying-spam-from-your.html

Test carefully if you choose to implement a method like this.

Or you can add a tag to the header for outbound mail and only accept NDRs if the tag is included. Unfortunately the blog I first found that one in (a few years ago) doesn't appear to be responding.

http://taint.org/2007/05/30/164456a.html

As with the right modification above it will need careful testing.

Chris

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mchad65Author Commented:
Thanks for the detailed response.  We are still using 2003, so that's not an option at this time.  I am willing to accept is as a fact of life on the internet, I just wanted to make sure the SPF was properly formatted.  Sounds like it is (from other sources as well).  So I am willing to live with it.
Chris DentPowerShell DeveloperCommented:

The only part you may consider changing is ~all to -all. That switches you from soft-fail to hard-fail (reject completely rather than tag).

I doubt it will make a difference really but there's little harm in it.

Chris
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mchad65Author Commented:
What is meant by "tag"  and, is there any risk at all by changing to hard-fail?

Thanks
Chris DentPowerShell DeveloperCommented:

Tag it so it has more chance of being binned as Spam. The risk with hard-fail only appears if you forgot to add a server or two to your SPF record.

Chris
mchad65Author Commented:
Perfect.  Thank you!
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